guru is one who dispels ignorance.
Most of us long for happiness and abhor misery. We exert ourselves for this, though we may not succeed in doing so. To tell us that such happiness does exist, teach us the way to it and to get us the grace of God to bring our efforts to fruition, it sometimes becomes imperative to seek a guru's guidance.
The Bhagwad Gita gives the qualities of a shishya, student, as well as the attributes of a guru.
A shishya must have three traits: Pranipata, surrender; Prashna, question; and Seva, service. First, we need to acknowledge our ignorance, and then seek knowledge from a wise person.
The guru helps us look within. We are so attached to and involved in our own lives that we are unable to view things dispassionately.
The guru dispels the darkness of ignorance and leads us to the light of realisation. He helps the student destroy all the obstacles that obstruct the path towards transformation.
What is the difference between a teacher and a guru? A guru is one who himself is the light. The last words of the Buddha were also the same: Appo Deepo Bhava, be a light unto yourself.