You can read the Apology online in English at the Perseus Project and in French at Wikisource. John Sellars has a good article on the Apology as a metaphilosophical text -- that is, as a philosophical examination of the idea of philosophy itself.
There are two speakers:
Meletus is the primary accuser of Socrates; he is a young man. His father, or a man named Meletus who was very likely his father, is well attested in other sources as a poet and tragedian, but Meletus himself is portrayed as relatively unknown, and nothing definite is known of him that does not trace back either to Plato or to Xenophon.
There are two co-accusers who are present but do not speak, although they are much better known and influential than Meletus:
Anytus is a speaking character in Meno, in which he displays a vehement and almost irrational distaste for anything even suggestive of sophistry, which no doubt is linked to his participation, given that Socrates was often lumped in with the Sophists.
Lycon is not elsewhere a character in Plato's works, but he is a character in Xenophon's Symposium, and Xenophon -- who, despite many admirable qualities, is entirely capable of holding a grudge -- seems to regard him very favorably; there is never any suggestion of criticism of him in Plato, either. Xenophon shows him as very, very close to his son, Autolycus, who was killed by the Thirty Tyrants. Debra Nails in The People of Plato suggests that Lycon's participation may well be due to Socrates' connections with Critias. There was a general amnesty on the point, so that events of that time were not supposed to be causes of prosecution -- but an official ban on public motivations, of course, can do nothing whatsoever about private motivations.
In addition, there are a great many others, the whole jury and then an unknown number of spectators. Some people are specifically mentioned by Plato as being in attendance: Chaerecrates (Chaerephon's brother), Crito, Critobulus (Crito's son), Aeschines and his father Lysanias, Antiphon of Cephisia and his son Epigenes, Nicostratus of Athmonon, paralius (son of Demodocus), Plato and his brother Adeimantus, Aentodorus of Phaleron, and Apollodorus of Phaleron.
In understanding the context of the defense speech, there are certain things that are useful to keep in mind. (1) Athenian juries were very large; there could be as many as 1500 on a jury. The jury for Socrates' trial was about five hundred people. Jurors were chosen by lottery from a pool of volunteers, and were required to swear the Heliastic Oath. (2) They were productions for an audience -- especially for the jury, of course, but spectators were also allowed. The closest counterpart to a trial in ancient Greece is drama, and any number of things could be done that we would never allow in a trial at all. Athens was a democracy: trials were governed not by a principle of presenting evidence but by a process of trying to sway votes. Both accusers and defendants directly interact with the audience, as well, and there is no judge to keep the jury in line; at several points Socrates will have to ask the jury to quiet down and hear him out. (3) Athenian jury trials were not deliberative -- the jury would not retire for deliberation but simply vote on the basis of the cases presented.
The trial would last for about nine hours or so, divided into different parts carefully timed by a water-clock. After a reading of the charges, the accusers would have three hours to argue their case. Then Socrates would have had three hours to counter. The jurors would then vote by dropping tokens into urns to convey whether they thought Socrates guilty. The jurors only vote to convict by about thirty votes. Once he was convicted, accusers and accused would each get to propose a punishment, and the jury would then vote between the two punishments proposed. In Socrates' case, of course, the accusers propose death, and Socrates, while saying that his punishment should be free meals like an Olympic victor, eventually proposes a fine. They vote, of course, for the death penalty. Plato doesn't give us any exact numbers for any of the votes.
After remarking that the Athenians should not expect fine speeches from him, Socrates goes on to suggest that he is in fact fighting against more accusers than there are in court; his real opponent, in other words, is not Meletus, Anytus, or Lycon, but the fact that many of his jurors know him only from misleading sources, and thus already come with a set of prejudices against him:
There have been many who have accused me to you for many years now, and none of their accusations are true. These I fear much more than I fear Anytus and his friends, though they too are formidable. These earlier ones, however, are more so, gentlemen; they got hold of most of you from childhood, persuaded you and accused me quite falsely, saying that there is a man called Socrates, a wise man, a student of all things in the sky and below the earth, who makes the worse argument the stronger. (18b)
Socrates notes Aristophanes in particular, for his representation of Socrates in The Clouds, but it is not Aristophanes alone; everyone who has carried this rumor about Socrates has been one of these accusers from of old. Socrates categorically denies that any of it is true. Why, then, did the rumor arise at all? And thus we get to the Delphic Oracle.
Chaerephon, who is dead but whose brother Chaerecrates is in the court, went one day to the Oracle at Delphi and asked whether any man was wiser than Socrates; to which the god Apollo, speaking through the oracle, replied that there was no one wiser than Socrates. This, Socrates says, is the beginning of the practice for which Socrates was known. Interpretation of the Oracle was notoriously difficult, so Socrates set out to find someone wiser than himself. However, whenever he came across some purported expert and started asking him questions, Socrates soon discovered that the 'expert' did not know as much as he thought he did. Thus, Socrates says, he saw that, for all the alleged expertise or skill of that person, Socrates was wiser than he, because at least Socrates did not think he knew things he did not know.
It is to this uncovering of the ignorance of experts that Socrates attributes his unpopularity; but he could do nothing else if he were to take the Oracle seriously. He investigated politicians (cp. Laches), he investigated poets and rhapsodes (cp. Ion), he investigated craftsmen (cp. Xenophon's Memorabilia 3.10). And in every case the discovery was the same. Thus, says Socrates, what the god likely meant was that human wisdom, in comparison with divine wisdom, was as nothing, and that he is humanly wise who, like Socrates, recognizes the relative worthlessness of his own wisdom. And it is because some of the youths around him began to imitate him in his task that there have grown up rumors that he corrupts the youth.
So much for the general accusers. He then addresses the accusations of Meletus by entering into a dialogue with him. He has no difficulty tying Meletus up in self-contradiction; while Meletus claims that Socrates disbelieves in the existence of gods, he also attributes to him views that presuppose the existence of gods.
Having handled the accusations themselves, Socrates goes on to consider whether he should be ashamed to have been involved in an activity that put him in such danger of death (cp. Gorgias, in which Socrates also addresses exactly the same argument). Socrates replies, however, that when a man is placed where he should be, it is a disgrace for him to run away out of fear of danger and death, just as it would have been shameful for him to run away from his post when he fought for Athens at Potidaea and Delium and Amphipolis. And, as we have just seen, Socrates was given his role by the god Apollo, and even if the jury were to offer to acquit him on the condition of never practicing philosophy again, he would obey the god rather than men: he would continue to point out to the people of Athens the importance of looking after their own souls and cultivating virtue. If Socrates is corrupting the young, it is this message that must be harmful; but that is the only message he has ever had.
Socrates' defense speech, he says, is not so much a defense of Socrates himself, but a defense of Athens. Meletus and Anytus cannot truly harm Socrates by punishing him for doing right, but if Socrates is doing right he has all the good that is actually important. But Athens is on the verge of harming itself by doing a great wrong, "mistreating the god's gift to you by condemning me" (30e). Yes, Socrates does literally call himself the god Apollo's gift to Athens.
If it seems strange that Socrates does not seek an active public life but does all his work privately, it is because he has, as Meletus mockingly noted in his deposition, a divine sign, a daimonion or daemon-like something:
This began when I was a child. It is a voice, and whenever it speaks it turns me away from something I am about to do, but it never encourages me to do anything. This is what has prevented me from taking part in public affairs, and I think it is quite right to prevent me. (31d)
He gives his role in the Arginusae affair as an example, when Athens was a democracy, and also his refusal to cooperate with the Thirty Tyrants in their attempts to execute opposition, when Athens was an oligarchy: "Then I showed again, not in words but in action, that, if it were not rather vulgar to say so, death is something I couldn't care less about, but that my whole concern is not to do anything unjust or impious" (32d).
Socrates gives the examples of several upstanding citizens who have associated with him and not in any way been corrupted, and then goes on to scold anyone who is thinking of voting for his guilt simply because he doesn't engage in elaborate courtroom dramatics: "It is not the purpose of a juryman's office to give justice as a favor to whoever seems good to him, but to judge according to law, and this he has sworn to do" (35c).
The guilty verdict is returned, and Socrates argues that if he were given the 'punishment' he deserves, he would get free meals in the Prytaneum just like victorious Olympic athletes do. But since they will not take that sort of recommendation, and since he has no money, he will give the assessment that Plato, Crito, Critobulus, and Apollodorus have suggested they could guarantee, thirty minae of silver. (A mina is 100 drachma, and 1 drachma is a standard day's wage for labor. So the fine is actually quite substantial.)
The death sentence is returned. Socrates addresses first those who voted for his condemnation and then those who voted for his acquittal. Those who voted for his condemnation he rebukes, because they voted as they did because he would not pander to them, and while he may be condemned to death by them, his accusers are condemned to wickedness by truth itself. To those who voted for his acquittal he says that his daimonion did not prevent him from defending himself as he did in any way, and "it is impossible that my familiar sign did not oppose me if I was not about to do what was right" (40c). He also remarks that death is perhaps not such a bad thing; if it is complete lack of perception, then this would be no more disadvantageous than a good's night sleep, and if it is a transition to another place, surely since no one escapes it, it is better to arrive before the judges of the underworld just than unjust. (Cp. Gorgias again, which makes exactly this argument as well.) And he will be able to talk with Palamedes and Ajax and others who were unjustly put to death, and could spend his time examining people as he always has.
He ends by saying that he is not angry at his accusers, but says that they should take their revenge on him by treating him as he has treated them: if his sons grow up not to care for virtue above all things, they should cause the boys the same kind of grief Socrates caused them, rebuking them and insisting that they focus on the right things. And then we have the famous ending lines:
Now the hour to part has come. I go to die, you go to live. Which of us goes to the better lot is known to no one, except the god. (42a)
Quotations are from G. M. A. Grube's translation in Plato, Complete Works, Cooper and Hutchinson, eds., pp. 17-36.