If it's a yellow plug it's probably an RCA port for composite video, the cables required are very inexpensive. The quality depends greatly on the TV. On an old CRT TV it's not great, as the resolution is really limited. On a HDTV, projector or similar it can look quite good, although obviously not as good as with component, DVI or even S-VIDEO connections.
To use it, just plug an RCA to RCA cable into the TV-OUT port of the video card, and into a video input port on the TV, and enable a clone display in your video card driver (making sure you have the nVidia driver).
I've used the TV-OUT port on my video card with composite video, and it doesn't look bad for watching movies on the TV (it's certainly better than VHS or VCD, that's for sure), but it's not great for games. Again, this depends on the TV.