All HDTV's conform to the new digital standard setup a few years ago. And all HDTV broadcasts now follow this standard. This includes all possible 'cable channels'.
So all modern digital televisions are 'cable ready'.
But the cable & sat companies want to offer you more services such as DVR features (wonderful), on-demand replay of recent shows for free (wonderful) and on-demand and pay-per-view services (not so wonderful).
These extra services are NOT part of the HDTV standard so the cable/sat companies want to put a small 'vending machine' in your home to let you use or see what else is available. So many of them require a set-top-box.
And it is not uniform. In 1 city - you have to have a cable box to even have cable service (I suspect this is the case in your town). In other cities - they send about 30 channels that mimic what you might get from an antenna down the coax so you can simply plug into the antenna input without a cable box. These are usually older cable systems.
Personally - I love the DVR services. I never have to remember an 'appointment' and schedule my life around the broadcast schedule.
Example: My wife and I like the TV show "Castle". We did not get to watch it 'live' on ... Sunday? but we watched the recording last night. At the end they mentioned there was a 2-night event with a second show. I checked and my DVR was in the middle of recording it for me. The 'vending machine' which I pay extra for takes care of special events like this.
It's wonderful - but I do pay extra for this service.
If money is dear to you - ask about 'Basic' service. They don't like to talk about this - but all cable companies offer this. During an Olympics I was paying about $29 for Time warner Basic service but it included a DVR box, all local channels and a bunch of other crap.
Hope this helps.