Bliizard calculates your crit chance based on your level, and an equal level mob. At level 5 you had a 10% chance to crit a level 5 mob, but when you dinged level 6 the game now assumes your crit chance based on a level 6 mob. With no changes to your gear, your crit chance went down because the new level 6 mobs have a higher defense rating whilst your attack rating remained the same.
That's probably not entirely right, but it's something like that. Somebody will school me shortly.
EDIT: Wow, two responses while I was still typing. As for your second question, check WoWWiki @ http://www.wowwiki.com
Edited, Nov 15th 2007 8:51pm by Mutalisk