Today is the feast of Ss. Philip and James. We actually know a fair amount about Philip: he was from Bethsaida, and seems to have been a disciple of John the Baptist. When called by Jesus, he went and found Nathanael and brought him, and is the second apostle, after Andrew, to claim that Jesus was the Messiah. He is always listed fifth of the Twelve, after the two pairs of brothers (Peter and Andrew, James and John), which perhaps indicates his importance, and he speaks several times in the gospel of John.
James the Less is somewhat trickier. He is not the brother of John (that James is sometimes called James the Greater, to distinguish him); and his epithet, which is Scriptural (Mk 15:40), should really be "the Little". What we know for sure is that his mother's name was Mary of Cleophas (and that probably means his father's name was Cleophas) and his brother's name was Joses, probably a form of Joseph. But there are three other cases where someone is called James who might be the same James.
(1) James son of Alphaeus, one of the Twelve Apostles;
(2) James the brother of the Lord
(3) James the brother of Jude
By tradition these are all four considered the same. If James the Less is not any of these, then we know next to nothing about him. But Paul in Galatians 1:9 seems to suggest that James the brother of the Lord was counted as an apostle, and it would explain why James the brother of the Lord had such extraordinary importance in the early church. James the brother of the Lord was the leader of the Church in Jerusalem. Because of this he is the Apostle to whom the later Patriarchs of Jerusalem traced their succession. He also seems to have been very Jewish (hence his other name, which, although first attested in the second century or thereabouts may have been widely known: James the Just, i.e., the Righteous, which seems to indicate close observance of the Law), and Eusebius notes that the Patriarchs of Jerusalem continued this, all of them being "of the circumcision" until Hadrian expelled the Jews from Jerusalem. It is interesting to reflect on how the Church might have been different had their been an unbroken succession of Jewish Christian Patriarchs in Jerusalem. In any case, Paul says that he, with Peter and John (James the Greater is martyred in Acts 12:2, and it's not clear what happened to Andrew), are the "pillars of the church", and it is to him that the Epistle of James is attributed. He seems to be explicitly mentioned by Josephus, who says he was stoned to death. Later legends say that he was first thrown off the Temple, but did not die from the fall, so was finished off by stoning.