"The Palace of the Lord God is so beautiful. Within it, there are gems, rubies, pearls and flawless diamonds. A fortress of gold surrounds this Source of Nectar. How can I climb up to the Fortress without a ladder? By meditating on the Lord, through the Guru,
I am blessed and exalted. The Guru is the Ladder, the Guru is the Boat, and the Guru is the Raft to take me to the Lord's Name. The Guru is the Boat to carry me
across the world-ocean; the Guru is the Sacred Shrine of Pilgrimage, the Guru is the Holy River. If it pleases Him, I bathe in the Pool of Truth, and become radiant and pure." (Guru Nanak, Sri
Rag, pg. 17)
The word 'Guru' in Sanskrit means teacher, honoured person, religious person or saint. Sikhism though has a very specific definition of the word 'Guru'. It means the descent of divine guidance to mankind provided through ten Enlightened Masters. This honour of being called a Sikh Guru applies only to the ten Gurus who founded the religion starting with Guru Nanak in 1469 and ending with Guru Gobind Singh in 1708; thereafter it refers to the Sikh Holy Scriptures the Guru Granth Sahib. The divine spirit was passed from one Guru to the next as "The light of a lamp which lights another does not abate. Similarly a spiritual leader and his disciple become equal, Nanak says the truth."
courtesy : www.sgpc.net