look at the back of your tv/cable box...that's what a tv tuner looks like except it will be sticking out of your computer
it will have a coaxial cable and probably audio and video input jacks as well.
i'm not sure about cable company's policies on computer TV tuners. they may treat it like another television requiring you to set up another cable line (ie more $$) or you can just use a splitter from your existing connection.
it will probably come with pre-installed software to control the channels, tvguide, recording, etc.