TV capture card

After reading Dissident's thread about the ABC GF DVDs, someone mentioned TV capture cards - I've never had much experience with these. Can anyone give some advice about them? How do they work? As I understand it, if I put a card in my PC I could copy TV shows to DVD much like a DVD Recorder - is it really as easy as that or are there pitfalls with doing that.
Any help would be appreciated.
Any help would be appreciated.