HD, has a resolution of 1280 x 720 pixels and is mostly found in smaller TV screens.
Full HD or FHD, has a resolution of 1920 x 1080 pixels, roughly twice the resolution of 720p but just a quarter of 4K.
I know you get quite a few discussions of this but mine is slightly different I guess. I've got a 20" Viewsonic LCD HDTV that supports 720p & 1080i. For now, I've only connected my Xbox 360 to it via ...