Most are HDMI video capture cards(without the ability to capture HDCP protected content) for capturing your video games. Some even let you capture to a flash drive or hard drive without a PC.
Like Fraps but in hardware. I think Fraps should have better quality, but requires a more powerful system.
Everything I've heard (which isn't much) is that Fraps really slows down frame rate which the AverMedia card does not.