The word Guru is Sanskrit for "Teacher," and comes from two words meaning "light" and "dark." Thus the literal meaning of "Guru" is "One Who Leads From Darkness Into Light."
It is important to understand that in Indian Culture a Guru is given tremendously high respect, much higher than is normally connoted by the word "teacher." So much so that the word Guru is often used as a metaphor for a supernatural entity such as one's Genius, Spiritual Guide, Holy Guardian Angel, Augoeides, etc.
On a more mundane level, Gurus are often the chief teacher or originator of a particular tradition.