At a 30,000 ft level, the 500, 550 and 600D models are the entry level units - physically smaller to hold, and with the various features accessed by menu options rather than physical buttons.
The size is important, as it's not always clear from publicity shots just how small these models are - and I find them harder to hold than a full size body. They also have more 'plastic' in their construction - and feel cheaper to hold/use.
50 and 60D are for the more serious photographer - you have buttons and dials to access features directly, and no-longer have to go through menus on the LCD to change things like ISO, WB etc etc
Size is larger - more of a normal body - much easier to hold and balance with a lens. (IMHO) Much improved build and feel - more solid and longer lasting.
7D is another step up - giving you 5D like features at a lower price point
Key features are:
18MP APS-C CMOS sensor
8 frames per second continuous shooting
1080p HD video recording with manual controls
3.0 inch Clear View II LCD screen with 920,000 dots
19-point AF system (all cross-type)
1.0x magnification and 100% coverage viewfinder
Wireless flash control
Environmental sealing
These are really pro-am features - allowing you to shoot successfully in a wider range of circumstances - the AF system and 100% viewfinder would be key for me, as they directly impact your ability to capture a subject the way you intended to see it.
A lot of the other features are around video, and I'm not sure if that's a big deal for you or not?
You really need to go hold the models physically in a store to see how you like them - after that, I'd go 50/60D or 7D to give myself the best tools for the job.
7D if funds allowed.
After that, it's a move to a full frame model, and the price that goes with that.
All of the models mentioned can produce excellent images, and use all of the main EF lenses.
I've used the Rebel (SWMBOs first DSLR), the 40D (SWMBOs current DSLR), the 60D (Son #1's current DSLR) - all to great effect.
