Recording direct from TV?

I know that this should be really simple....but; I have a plasma tv which gets a picture and sound from my pc graphics and sound cards. How, therefore, can I record onto my pc what is on the tv screen? Surely the same cables have the capacity to send the information back to somewhere?

Likewise, my DVD recorder should be able to record what is on the tv, (direct TV rec) but it only records what IT sends out, which is very misleading and annoying.(and it obviously doesn't record copy protected data!)