Buddhist devotions to bodhisattva?

I have a friend who is new to Buddhism. He is in love with the Bodhisattva Guan Yin. He has a deep devotion to it. No Bodhisattva Buddhist worship? How often these people worship? What is the difference between a Buddha and a bodhisattva? My friend is new in this so he really can not answer for me. I think he wants to join the Tibetan Buddhists. Thank you.

bodhisattva [Boddi sátvə] (bodhisattvas plural) the Buddhist deity n: in Buddhism, a deity or a being who has attained enlightenment worthy of nirvana but remains in the human world to help others [early 19th century. <Perfect knowledge''+ Sanskrit bodhi sattva "being, reality] Encarta ® World English Dictionary © & (P) 1998-2005 Microsoft Corporation. All rights reserved.

[affmage source="ebay" results="90"]Yin Bodhisattva[/affmage]