The low down:

Sensors:

This is where you see the differences, and where you need to do some study. The sensor is what digital cameras use instead of film; therefore it's crucial to chose the sensor and the camera as a package, don't just shop for the camera without considering the sensor that comes bundled in with it.

There's two main kinds, 'full frame' sensor, and the other. 'The other', is usually about 28mm, remember that film in SLRs was 35mm, and so a full-frame sensor is a full 35mm. Consider, the 28mm sensor is cheaper, and the full frame is more expensive. Now, which gives the best results? Well, usually the more you pay, the better it is. The big issue with 28mm sensor is noise, see the noise tutorial if you're not sure what this is. The basic rule is: bigger the sensor; the less noise there'll be in the image.

But, which sort of sensor is more common? The smaller one. Only the most expensive high-end cameras have a full-frame sensor, so whatever you buy, it'll probably be the smaller one. But these days, it's no major issue.

Types? Well, there's CMOS, and CCD. The CMOS seems to be better, and Sony, recently, got a good deal with the makers of the CMOS sensor, so their new A700 is simply awesome. Canon is the other major user of CMOS, and so their colour spectrum is fine, though the Nikon (CCD) and Konica Minoltas (CCD) had pretty good colour, but noise was a little annoying.

Mega Pixels:

Currently, all new SLRs are at least 8 mega pixels, and so you're in the 'good quality zone', anyway. Also, all new models, in order to compete, must be in the double digits, and so you're usually assured of excellent performance, and with much less noise compared to previous models.

So, mega pixels aren't really an issue in selecting a camera for you, these days.

Your style:

Essentially, this will help you work out which camera is best suited to you. If you're into landscapes, then anything is fine; if you're into travel, chose a smaller one like Sony A100, Canon Rebel, or Nikon 40; if you're into portraits, chose a Sony, as the colour for skin tones is better, if you're into sport, you can't go past the latest Canon or the Sony A700. Camera manufacturers are currently coming to the end of setting up their lines of digital cameras, and the market will be saturated with the second generation of digital SLR cameras, and the third generation up take is expected to be slow and a little more distant in time. I'm not sure how things will change after the peak change over period (from film to digital) is over. But things will probably look as good as they appear now for us photographers.

Below is a chart of major styles of photography, and the main features you could look for in a camera for that style. Each photographer will have their opinions, so don't hold this list like a bible.

* We mean 'mini studio' work     ^ For instance, flower photography

Explanation

More specifically

These are not endorsed, but consider these cameras in particular, when shopping: Olympus e500; any Sony Alpha, particularly A100 to A700; Nikon d40 to d80; Canon 400d (aka Rebel, aka Kiss), and others; Pentax k100d, k10d, k110d, the SLR Fujifilms, and Sigma.

Tutorials:

Some useful tutorials that relate to your style. Got questions? Ask on the forums.