A guru is not a teacher; The guru must himself be enlightened, he must himself have attained, because one cannot communicate that which one has not realised. 

A teacher need not be self-realised, but a guru must be. 

A teacher can give secondhand information from scriptures or traditions, but a guru cannot. 

A guru is a person who has realised truth. Now he is the original source; he himself has encountered reality, he is face to face with it. - Osho