Sikhism is often described as being a synthesis of Hinduism and Islam. That can be a simple way to understand what the Sikh tradition is, but it doesn't do justice to the rich spirituality and culture that the Sikhs have given to India and the world over the centuries.
The Sikh movement was founded by Guru Nanak in India in the late 1400s. Guru Nanak and the line of gurus that followed him taught that there is only one God, and that the goal of human life is to merge with God and be free from the cycles of reincarnation. Family life and community service have traditionally been emphasized, rather than the celibacy and renunciation idealized in Hindu practice. Caste distinctions are also rejected by Sikhs.
Sikhs describe four stages of spiritual evolution: Manmukh (the worldly person), Sikh (one dedicated to the spiritual path), Khalsa (one deeply committed to Sikhism), Gurmukh (one spiritually liberated and God-centered).
For Sikhs, long hair is considered a sign of spirituality and dedication. It is normally kept wrapped up in dastar or turban.
The holy book of the Sikhs is the Guru Granth Sahib, a collection of sacred poetry and teachings originally given by Guru Nanak, the early line of gurus, and other saints revered by Sikhs, including Kabir. The Guru Granth Sahib is now considered to be the "perpetual Guru" as there are no longer individual gurus who head the Sikh tradition. The sacred poetry and teachings contained in the Guru Granth Sahib is the final authority in Sikh spirituality and tradition.