Sound cards and outboard Analog-to-digital converters will process the signal and convert it into binary data. You can get away with the built-in soundcard in many cases. I've also used the Griffin Technologies iMic and the M-Audio Audiophile USB.
I've found that connecting an outboard converter gives an immediate jump in quality. I assume that this is because it's a dedicated system that is isolated from the fan noise and interference from the computer chassis, but I could be talking through my bunghole on this one.
I started with the iMic and was quite satisfied for a couple of months, then found it started adding a good bit of distortion. It's a very good cheap 'n' cheerful solution if you have only a few records.
The Edirol UA-1EX might be a good option for USB. ART has a phono preamp and converter with USB connectivity rolled into one that may be a good buy. Haven't tried it though...
If you're just converting vinyl, you don't need more than one channel, so you can get away with the lower end models. Prices rise spectacularly when you add channels and goodies that a small band might use for microphones, guitars, etc., but in your case, dear vinyl junkie, they're all sitting idle while your platter spins at 33rpm.
Here's a list of USB interfaces.
And a list of Firewire interfaces.