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.
A Guide to the Path
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, they give you information, test your knowledge and tell you if you are correct or incorrect. When practicing on the path to enlightenment your guru is your personal trainer, therapist and swami. He or she will provide you with the guidelines, listen to your woes, provide guidance and send you forward on your travels.
The basic word difference between Teacher and Guru is the difference between 'takes' and 'makes'. For instance a teacher takes responsibility for one's growth whereas Guru makes one responsible for their growth. Similarly one beautiful difference is wherein it says a teacher prepares you to answer the questions whereas a Guru will question your answer.
Do Not Blindly Follow
I have been asked why we do not chant or do a group invocation at Lake Tahoe Yoga. My reason is because I do not feel qualified to tell you which mantra is the appropriate one. I invoke that which I need and that which aligns with the intention of the practice as appropriate based upon that which I am developing and teaching. I encourage you join in at the closing of the practice to offer peace through the words, "Aum Santi," but do not expect you to do so.
As a teacher, I am still learning. My intention is to develop a strong grasp of the practice and philosophies, to develop them in my own life and be able to demonstrate and explain these practices to those who choose to practice with me. I am a work in progress and self discovery is still out of my reach.
Seek out guides who are sincere, authentic and offering their whole self to you as they teach. A guru they may not be, but they will provide you with a mirror to your self and your needs as you prepare to discover a guru.
Seek Knowledge and Comprehension
If your instructor cannot explain or describe that which they are teaching or the reason for it, then you probably should not be following their lead. Lineage, quality training, continued study and dedication to the practice are the ways in which we shed light upon the darkness. You don't need to know everything about everything, but knowing what and why you are doing something is very important.
Do not be fearful of asking questions of your instructor. Whether new to the practice or experienced in teaching, they should be able to answer questions about why you are doing particular poses, breath practices or mindful focuses and the intention behind the practice. If the answer is less than inspiring or lacks knowledge, then move on to someone who provides you with what you need.
The Ultimate Guide
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.