Someone please explain TV tuners to me

Alright, so here's my story. I'm a pretty knowledgeable tech guy, at least, I used to think so. In fact, I'm typing this on my custom built gaming system. Yay for me, right? Anyways, lately I've been interested in building an HTPC for my family's entertainment room. I've got all the parts picked out, all the connections and stuff, but one piece eludes me: the TV Tuner.

At first, I thought all it would allow me to do is schedule recordings, pause and resume live TV and such, but the more I read, the more confused I get. Currently, my family has a set top box from our cable provider, giving us standard definition channels. Within the next 6 months, we will be upgrading to HDTV, with another set top box. If I am to assume what I have been reading is correct, a TV tuner attached to that TV would not allow me to record anything through the box, but only what the TV Tuner picked up through the air. What is the point of attaching it to the TV then?

If anyone can clear this up for me, I'd really appreciate it. Thanks!

basically, a tv tuner takes the cable chord or whatever, and makes it so your computer can read the signal. you basicallyplug your cable into it instead of your tv.or with your tv if u have a dual split. its just a card to let u use tv on your computer, record, playback, hdtv whatever a tv can do.

Really? That simple? Well, I'm surprised. Anyways, I'm assuming QAM is a fairly common aspect of most cards now, am I correct? Thank you for putting to simply though, every other site just used confusing acronyms which did nothing in explaining