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 don't blame you for getting rid of your older 720p models, but if you have a few lying around, you might surprised how much value they have. No, you won't be selling them for hundreds of dollars, ...