Unless you have a ATI all-in-wonder (or other brand video card with tv tuner built in), you need a seperate TV tuner card. There must be some mechanism that receives TV coded signals (whether through rabbit ears or cable tv/VCR box) and decodes into computer talk (simplification).
Now many cards have a video "out" that you can hook to a TV, but very few have a TV signal video "in".
If you want to buy one to be able to watch tv through your PC, they are pretty cheap at about $40-50 bucks for a basic one like a ATI PCI TV tuner card. More expensive models have remote controls. They also generally have TIVO type capabilities in being able to record anything coming into the PC onto your hard drive. be aware that it takes up a LOT of hard drive space.
I have a basic ATI TV card and it works fine with either rabbit ears or cable/dish input (through a VCR). It comes with a nice nice program (forgot name) that allows you to download show listings of every channel without charge.
I basically got it so I could hook up my PS2 and play games and not hog the TV. Works good, but PS2 on nice monitors really shows how crappy PS2 games look.