About 4 or 5 years ago, all the broadcast TV in the US changed over to a digital format, and the digital format includes HDTV broadcasts. If you have an HDTV and an antenna, and you live in a place where you can receive the signals, you can get the HDTV of all the broadcast networks over the air (OTA) with no cable.
It has been reported that Comcast re-compresses the digital HDTV streams, cramming them into a smaller digital channel in their cable system, in order to fit more channels in. This leads to reduced quality in the picture you view on Comcast compared to the OTA HDTV broadcast. I don't know about other cable systems. Here is one such report, though it seems to be specifically about other non-OTA HD channels (where the FIOS broadcast was used for comparison).