Sikhism began around 1500 in Northern India, now the country of Pakistan.
Guru Nanak.
The Adi Granth, revered as the present guru of the faith.
Estimated at nine million, mostly in India's state of Punjab.
Besides the Khalsa, there are the Ram Raiyas in Uttara Pradesh and two groups that have living gurus - Mandharis and Nirankaris. The Khalsa is no sect. The Khalsa are the followers of Guru Nanak through Guru Gobind Singh. Nirankaris and Namdharis evolved in the 19th century / beginning 20th century as a reform movement. Besides straying from the original teachings they began to worship a living Guru which was against the order of the last Guru(Guru Gobind Singh). Radhasoamis are a group gathered by the promise that they may do everything and still be good Sikhs, additionally they are given secret mantras, which they may not tell anyone, not even their spouse.
The Muslims began their invasions of India some 1,200 years ago. As a result of Islam's struggle with Hindu religion and culture, leaders sought reconciliation between the two faiths, a middle path that embraced both. Sikhism (from sikka, meaning "disciple") united Hindu bhakti and Sufi mysticism most successfully. Sikhism began as a peaceful religion and patiently bore much persecution from the Muslims, but with the tenth guru, Gobind Singh, self-preservation force a strong militarism aimed at protecting the faith and way of life against severe opposition. Sikhism stresses the importance of devotion, intense faith in the guru, the repletion of God's name (nam) as a means of salvation, opposition to the worship of idols, the brotherhood of all men and rejection of caste differences (though certain caste attitudes persist today). There have been no gurus in the main Sikh tradition since Guru Gobind Singh, whose last instructions to followers were to honor and cherish the teachings of the ten gurus as embodied in the scripture, Adi Granth.