Guru (Sanskrit: गुरु:  guru) is a Sanskrit term that connotes someone who is a "teacher, guide or master" of certain knowledge.

GU- DARKNESS ; RU- LIGHT
GURU is someone who removes darkness and fills in light in your life.

           Guru Mantra

Gurur Brahmaa Gurur Vishnu
Gurur Devo Maheshwarah
Guru Saakshaata Parabrahma
Tasmai Shri Guruve Namah

Meaning of Guru Mantra:

Guru is Brahma, Guru is Vishnu,
Guru is Maheshwara (Shiva),
Guru is Supreme Brahman(Ultimate Reality) Itself
Prostration unto that Guru

{Brahma(creator); Vishnu(Preserver); Shiva(destroyer), are the three gods in the Hindu mythology}