The Sanskrit word guru means 'remover of darkness and ignorance'. It is a negative term in that it does not say that the guru gives you anything, only that he removes something. He removes the mental blocks and obstacles that prevent the spontaneous effulgence of the light of truth. The guru's role is to remove the veil, the ignorance, the preconceived ideas and mental conditioning that prevent the already existing light from shining through. The more this accumulated dross is removed, the greater will be our awareness of us and existence.
The means adopted by the guru varies greatly. It can be by personal instruction, by inspiring his followers, by rebuking his disciples for actions which are not in accordance with their nature and by guiding disciples so that their actions are appropriate to their own physical and mental make-up (dharma). But whatever the method used, the aim is the same: higher awareness.
The guru is an essential part of life. His role is the essence of all mystical systems. As soon as a mystical system discards or loses the guru tradition, then it no longer remains mystical. It does not bring results. It retains all the laws, rules, formalities and rites; it retains the structure or the shell. But it loses the essence or the moving spirit behind it. This essence is the light of the guru. It is he who opens the disciple's eyes to the impossible.
Actually everyone we meet in life can be regarded as our guru because each person can teach us something. Even our enemies can be our best teachers, for they clearly highlight and show us our greatest problems, the obstacles to higher awareness. When we know our mental complexes, the deeper ones of which we are not normally aware, then we can take steps to remove them.