There are three types of TV tuners.
1) NTSC - Receives analog OTA signals or, possibly analog signals from a cable provider. Forget digital and HDTV. These are the same tuners you watched "I love Lucy" on when it was not in reruns. You can get by with a yellow composite cable to a TV's composite input.
2) ATSC, or a "digital" OTA tuner. - These receive OTA digital signals. If they are built into a HDTV, you can be assured that it will deliver digital AND HDTV signals to your set. You connect this directly to a TV antenna, not a cable box output.
If you purchase a stand-alone OTA ATSC tuner, be aware that not all ATSC tuners will pass a HD signal to your TV. Caveat Emptor - read carefully before you buy. All spiders are bugs but not all bugs are spiders.
If you have a HD tuner you'll want to use either the RGB component or HDMI output if you have 'em, depending on what the box offers but I don't believe there's any reason the RGB input won't bring out the best it has to offer. There's no OTA 1080p signals being transmitted.
Don't expect OTA to deliver the quantity of channels that cable offers, though.
3) QAM - These are made for digital cable and how well they work with your local provider is anyone's guess but out here they are virtually useless. Some cables companies won't initialize/activate any tuners/cableboxes except their own. Not much Ican addto this except that if it works and yourer cable company allows it, ist should replave your cable box, albiet perhaps a few interactive features suchas menus, on- demand and whatever else they don't do.
Personally, I have my OTA antenna hooked up to my TV's HD tuner input for local programming and my cable box to a component input for, duh, cable programming. They get along great.
Oh, odds are that if you connect the cable boxes's 75 ohm "F" connector to anything, it'll be spitting out a low rez modulated analog RF signal on channel 3 or 4, just like cable boxes did in the 70's.