I think High Definition eady and Compatible both mean the same thing: The display device can accept and sync to a "high definition" signal from an outboard source- but do not contain an internal HD tuner.
This just means you'd need an outboard HD tuner to get over the air HD feeds... or a HD source like a DirectTV box or DVHS machine.
Vince is correct. HD-ready, HD-compatible, and HD-upgradeable all mean the same thing: the unit will accept and display high def signals when fed decoded HD material. if you buy a unit that is HD-ready, you will need an outboard HD tuner or decoder in order to see the full HD signal, since it is not built in. i believe the tuner inside the TV only tunes NTSC.