Need advice for Professional Camera Purchase

[Babbling Bryan]

Ok I'm almost completely happy with the way my materialistic side of life is at the moment. I have a iPod video, a Macbook, a Wii, a Nintendo DS and a Canon Digital Ixus 60.

It's just the last one in that list that I would like to improve.. I really love taking pictures and would love to have one of those big reflex cameras like one from the Canon Eos series.
My spending limit is like 500 bucks.. but preferably less, second hand versions is also a option.

So what are my options, what are the better models etc?
I also know little to nothing about the professional camera's so what are the advantages about such a reflex (mirror reflex?) camera and what are the down sides (in comparison to my Digital Ixus, and please keep it in english, if you tell me that it has a shutter of 30 seconds or something I will not know what that actually means for my picture taking).

Thanks in advance.