There's this new technology where you can get your local networks via an antenna such as this one.
Haha, good stuff.
OP, you seem remarkably unfamiliar with the basics of television
🙂. Congratulations. No, really. It probably means you have spent your time more wisely.
What you can get "over the air" with an antenna is the basic broadcast stations in your local area. In practice this means ABC, NBC, CBS, maybe a PBS station, and maybe an independent or two. 10-12 channels if you're lucky.
I have an ATI tuner in my rig, with Comcast's coax plugged into the back of it. What I can receive with this setup is about 75 channels of "basic cable." These are the channels that Comcast provides in unencrypted analog radio frequency form. Just like taking the old broadcast television technology and instead of sending it out over the air, you fire it down a shielded wire. Comcast provides these channels in the analog format so that they can be received by people with just analog tuners: those with cards like mine, "cable ready" TV's and no set-top box, etc.
Comcast also provides digital cable. These channels are sent in a digital format (probably MPEG2) and encrypted using something called QAM. In order to receive these channels you need either: a) a set-top box from the cable company; or b) a QAM-capable tuner card (there are a few out).
Lastly, Comcast provides a bunch of premium channels that are part of add-on pay packages. I think these are also QAM encrypted digital streams, but furthermore they are not even transmitted down your pipe unless you are paying for them.
That's a pretty inexpert view of your choices. The guys in the Video Forum here know a lot more.