Granth is Punjabi for book; Sahib is Hindi meaning master, from Arabic, meaning companion, friend, owner, or master. It is the religious holy scripture of the Sikh faith, but in actual fact, it is considered to be more than just a holy book. The Sikhs treat this Granth (holy book) as a living Guru. The holy text spans 1430 pages and contains the actual words spoken by the Sikh Gurus and various other Saints from other religions including Hinduism and Islam.
The Adi Granth is often used to refer to the Guru Granth Sahib. The Adi Granth only forms the portion of the Granth which Guru Arjan compiled in 1604. This term is often used interchangeably so it is important to note the context within which it is used.
The Granth was made a guru by the last of the living Sikh Masters, Guru Gobind Singh in 1708. Guru Gobind Singh said before his demise that the Sikhs were to treat the Granth as their next Guru.