The one who knows and who has realized the truth. The one who knows the scriptures and who has direct experience of the knowledge. The one who dispels darkness and breaks the shackles of bondage. Such an enlightened, liberated being is a Guru.
Guru is like a sun who brightens up the dark ignorant mind. Guru is like a soothing moonlight which cools and soothes the pains and aches of the mind. Moon is also a symbol of complete enlightenment. In Buddhism, a full moon symbolizes Lord Buddha. Guru is like a full moon with all its lustrous beauty that brings life to a disciple and is ever ready to bestow wisdom on those who seek.
Guru is like a ladder. You are standing on the ground, but you wish to reach the top of a tree. You are at the starting point and you wish to reach the ultimate destination. So, Guru is the conjoining ladder between the two points. Guru sees himself or herself in you. For a self-realized person, there is no duality.
Guru is like a friend who helps you at times of need. Guru is the only person who can guide you through all the travails of life. Guru teaches you how to deal with suffering and loss. Guru guides you through the labyrinth of this life and helps you to reach the ultimate destination of human evolution i.e. self-realization.
Guru shows you who you are. Guru guides you from darkness to light. And at the culmination of this journey, you realize that you and guru are one.