Hello there Mr. Cat,
Perhaps you will enjoy this explanation of the mani mantra by His Holiness the Dalai Lama.http://www.sacred-texts.com/bud/tib/omph.htm
In my layman's terms, I think of "Om" as the holy syllable for beginning mantras and I see "Hum" as the holy syllable for ending mantras, sort of like how you'd try to put a holy picture in a beautiful frame. A lot of mantras begin with "Om," "Om Ah," or "Om Ah Guru" and end with "Hung" and just insert the deity's name in between: "Om Ah Guru Vajradhara Hum," "Om Vairocana Hum," etc.
There are many forms of Chenrezig in Tibetan Buddhism. He can can be red or white. He can have 2, 4, or 1000 arms. I'm sure there are still many variations on his form that I don't yet know about and I'm sure that other schools of Buddhism in Japan and China have there own styles of depicting him.
If you are interested in practicing the visualization of Chenrezig, it is best to seek the instruction of a qualified teacher.
This is certainly true, but I've read Tibetan lamas say many times that the practices of Green Tara, Chenrezig, and Medicine Buddha can all be practiced without first receiving empowerments. However, I have heard some lamas say that one should not visualize oneself
as the deity if one has not received the empowerment. You can instead visualize the deity outside yourself (usually floating above your head or in the space in front and above you).
Here is a nice little description of a Chenrezig practice which could be done without an empowerment:http://www.kagyu.org/kagyulineage/buddh ... /dev01.php
You can also recite the mantra while holding a mala and moving to the next bead after each recitation. In this way, you purify your body. The mantra purifies your speech, and the visualization purifies your mind.
There is no danger in doing basic Chenrezig practices. You will only create blessings for yourself and other beings.
Good luck with your practice, Catmoon.
OM MANI PEME HUNG