An 'HD ready' TV can display HD pictures but it doesn't have a suitable TV tuner (capable of receiving HD signals) built into it. To view pictures in HD you need to use a secondary device, such as a BluRay disc player, a 'Freeview HD' set-top box, a 'FreeSat HD' set-top box or a 'Sky HD' box.
Some TV sets which are described simply as 'HD' might actually be only 'HD ready'. (Some manufacturers don't make their labelling clear). If you want a (true) HD TV which can (without an external box) receive terrestrial TV in HD, you need to look for the 'Freeview HD' label on the packaging or product description, rather than simply 'Freeview'.
Similarly,if you want a TV which can receive satellite TV in HD, without an external box, you need to look for the 'Freesat HD' label, rather than simply 'freesat'.
Higher resolutions are better but you should also look at the screen refresh frequency. For example, if you're watching a football match, there will be much less 'motion blur' (as the camera pans from side to side) with a 600Hz set than a 50Hz one.
Chris