What does a tv tuner video card for my desktop do?

I have a big screen tv and want to be able to use it as a computer monitor. I have been looking on ebay for a video card and have run across some that say the have a tv tuner. What do these cards do? I want to put one in my desktop. Does this mean I will be able to watch cable tv on my computer or what. What does a tv tuner card do?|||Yes, with a TV Tuner card, you could watch TV on your PC & save video (with help of software).