The word Guru is of two parts: "Gu" meaning darkness and "Ru" meaning light.
In the world of spiritual endeavors the guru is the guide who helps your find your way on your path to enlightenment.
Unlike a teacher, a guru is there to help you along without giving you the answers. Think about it this way; in school you have lots of teachers, the give you information, test your knowledge and tell you if you are correct or incorrect. When practicing on the path to enlightenment you guru is your personal trainer, therapist and sherpa. He or she will tell you the guidelines, listen to your woes and send you forward on your travels.
As you move forward through your practice of Yoga you will likely follow many teachers and be guided by many gurus. The ultimate guru is the one that is within you. Like your conscience, it will tell you if things are right for you. Above all, no matter what your teachers say, you must listen to your inner guru if you desire spiritual enlightenment.