A TV with an ATSC tuner has to be able to display or process if you will all 18 formats available on the ATSC standard (yes there are 18 formats but the TV industry uses only 2, 720p and 1080i). As a matter of fact there are 2 types of digital TV's , the HDTV-Ready kind that basically can display those formats but lacks the ATSC tuner and the HDTV that therefore has the tuner incorporated. All recently manufactured and sold TV's in US are HDTV (with the tuner) as it was mandated by the FCC few years ago.

I am almost 100% sure the OP has therefore an HDTV fully capable of displaying the before mentioned 720p and 1080i formats hence enjoy your shows in HD from now on. Some TV's are 1080p nativelly therefore could display that format (1080p) that is used by the Movie industry with the Blu-Ray discs (very recently Pay TV providers like DN in USA are offering 1080p on demand content as well). But beleive me, you will see no difference , at normal viewing distances, watching 720p or 1080i content on a non native or a native 1080p display.

BTW , as usual , I almost forgot , Welcome to the forum. Enjoy it.