There are several options.
Eskape labs makes a TV tuner with a USB interface and software so you can watch TV on your Mac.
EyeTV turns your Mac into a digital TV recorder a al TIVO.
Formac makes a FW video digitizer with TV tuner. It produces DV format files. These are rather large but are useful for some purposes. It is also rather expensive.
There may be some others if you shop around. Try Google. Try calling an Apple store if one is near you. You might also try Apple's web site. From the main Apple page click the menu heading Made4Mac.
