There are three different types of resistance, these being:-
1.) Air - the back wheel attaches to a fan so the faster you pedal the greater the resistance. These tend to be the cheapest models on the market but also the noisiest so if you live with anyone or are joined to your neighbours by thin walls I'd give these a miss.
2.) Magnetic - you'll be spinning magnetic discs with the resistance being provided by other magnets that can usually be moved closer to or further away from the spinning disc to give variable resistance.
3.) Fluid - these work by multiple discs being spun within a fairly viscous hydraulic fluid and are said to be the closest thing to riding out on the road with resistance being varied by using your bike's gearing. These are usually the quietest of the three types.
Some have a computer included as part of the package but if you already have a bike computer with a rear wheel sensor you're better off buying the non-computer model as the mechanical working will be the same and you'll get the same workout.
Also worth buying are a riser block for your front wheel (or just use a yellow pages) so you don't feel like you're riding down hill all the time without the benefit of being able to freewheel and it'll take some of the weight off your wrists and you'll need some form of sweat catcher that attaches to the handlebars and seat post.
Elastogel is Elite's brand name for a polymer coating they put on their rollers that gives good grip for the back tyre, quiet operation and a long life.
Off the top of my head, some of the major brands are Elite, Tacx, CycleOps and Minoura but I'm sure there are others.
I have an Elite with fluid resistance and an elastogel roller and while it's very good at what it does and is very quiet, turbo training is perhaps the dullest thing imaginable (with the exception of Big Brother) so I'd recommend setting it up in front of the TV / DVD player and sticking a few box sets of your favourite TV programmes on your Christmas list too!