Capture Card Basics
There are basically five types of video capture systems; Analog M-JPEG, Analog MPEG, DV, Analog/DV combinations, and the rest (some using proprietary CODECs such as Indeo, and some ultra-high-end systems costing many thousands of dollars). These can also be broken down into realtime and non-realtime (although they are actually referring to how transition effects are done for output, not how they capture video in the first place). Note that when we say analog here we're referring to the type of video input signals not the end results (although some boards can't output back to a VCR without help).
All capture systems are designed to accept certain types of video input - PAL, NTSC, SECAM, composite, component, S-Video, DV, etc. so you need to make sure your video capture board matches your source.
Some capture cards have an external breakout box (or put all the circuitry in the external box). Breakout boxes are handy since they alleviate all that climbing behind your computer to plug things in and out - and believe me, you'll be doing a lot of that.
All capture systems have a maximum resolution and maximum frame rate. Some will also have a maximum data rate. Less expensive systems will have a maximum resolution of 320 x 240 or less but keep in mind that you can always make an image smaller if you have to but you can't expand a small image too far without losing a lot of quality so you should probably stick with a system that can handle at least 640 x 480 or 720 x 480. For the best quality you should also look for a system that can capture 60 fields per second rather than 30 frames per second. As far as maximum data rate goes you should look for a system that can handle at least 3.6 MB/s.
Analog M-JPEG boards were the first pioneers in digital video capture and have gradually lost out in favor of MPEG and DV alternatives but you can still find them out there (at very good prices) and they do a pretty respectable job.
Analog MPEG systems come in two flavors: MPEG-1 and MPEG-2. If you're serious about video (or think you might be sometime in the future) then it's worth it to go for the higher-end MPEG-2 systems - who knows, you might want to put your videos on DVD someday. MPEG-1 systems can be perfectly adequate but the quality can vary quite a bit from system to system.
DV capture systems are a special case and require a DV camera or DV VCR for input. These systems can also be divided into two categories; internal and external. In this case we're talking about where those proprietary DV encoder chips are located. Some early DV systems used the chips in the camcorder to process the video. They would capture the DV video stream from the camera, convert it into another video format such as AVI and then send the video back to the camcorder in order to perform the DV encoding. They were a little awkward at first but they've improved over the years.
DV capture systems have an advantage in that many of them can connect to and control the DV camcorder or DV deck remotely via IEEE 1394 connections. This is handy for batch capturing and editing.
Some DV capture systems have an unique feature that takes advantage of this external control. Rather than capturing and storing a DV signal on the hard drive they capture in compressed AVI (or another format). After you do all your editing and press the render button, the system goes back out to the camcorder or deck, grabs only the bits and pieces necessary, and outputs straight back out in DV format. Of course, this requires that your DV capture system knows how to control your particular DV Camcorder or deck. So if you have a DV camcorder it's important to make sure the capture system supports that particular make and model.