Sikhism, the fifth largest religion in the world (after Christianity, Islam, Hinduism, and Buddhism), is nonetheless virtually unknown to many people in the West; even (rather surprisingly) to people who regularly interact with Sikhs (there's a strong Sikh presence here in Toronto). A primer:
Sikhism originates with Shri Guru Nanak Dev Ji (1469-1538) in the Punjab (more precisely, in what is today Pakistan). Nanak began his preaching career on receiving a vision to preach enlightenment and the way to God. The religion Nanak founded is strictly monotheistic and has a strong emphasis on human brotherhood. After Nanak there were nine more Gurus, each considered a reincarnation of Nanak himself. There are, however, no more Gurus for Sikhism today. Or, rather (since Sikhs themselves would not put it that way), the Eleventh and Perpetual Guru of Sikhism is Shri Guru Granth, the primary holy text of Sikhism, consisting of the writings of the first nine Gurus and an occasional Hindu or Muslim text. You can find the Guru translated online.
A common cultural mark of the Sikh is the five K's: the Kanga (a wooden comb symbolizing order); the Kachha (undergarments symbolizing purity); the Kara (a steel bracelet symbolizing absolute service to Truth); the Kirpan (a sword symbolizing the defending of the right and the struggle against the wrong); the Kes (uncut hair and beard symbolizing respect for God, who gave it; it is always to be bound up in a turban, symbolizing holiness). All baptized Sikhs vow to wear these marks or memorials. Morally, the three main ideas appear to be 1) constant meditation on God; 2) earning an honest living; 3) compassion. Truth and truthful living is the binding thread of the three; 'Sikh' means 'one who learns', and Truth is perhaps the central element of Sikh belief. The root verse of Sikhism is (roughly, in English):
There is only one God;
His name is True;
He is the Creator.
Timeless and Formless,
Beyond birth-and-death, the Enlightened,
He can be known by the Guru's grace.
One of the distinctive features of Sikhism is its attempt to mediate between Hinduism and Islam. While Sikhism rejected the caste system and polytheism from the beginning, and does not exist to serve as a bridge between the two, it nonetheless has always been in some ways an attempt to find a peaceful middle ground. It does this, one might say, by proposing that everyone go behind their specific forms of worship to recognize the meditation on God, beyond any particular name, as more fundamental. As Nanak famously said, "There is no Hindu, there is no Muslim." As one might expect, this world being the way it is, the Sikhs have nonetheless had a long history that at times has been both bloody and painful. When the partition between Pakistan and India was made, part of the reason was religious; unfortunately for the Sikhs, though, the division was between mostly Muslim Pakistan and mostly Hindu India; the Sikhs were left out of the equation entirely. There are still Sikhs who hope for the re-establishment of the old Sikh state of Punjab, in some form, in some form, at least. That state was destroyed by British invasion in the 1840s, as the British extended their sway over all India.
All About Sikhs is a useful website for basic information.