Yellow rares are elites, and have a bunch of weaker 'minions' with them who don't necessarily take on all of their special attributes, where as the purple/blue ones are champions, and each one of them in any given group is the same strength as all the others in the same group.
killing the elite will net you the good gear drop, whilst you need to kill all of the champions in a pack to get the good gear drop.
edit: sorry I just understood what you meant by purple. the purple ones are the 'uniques'. they have special names (not random words thrown together) and their subtitle isn't just a list of affixes, but rather a bit of flavour text not pertaining to what affixes they have (some don't have any special features apart from doing more dmg and having more health)