There's a lot of confusion about this, not helped by the average sales person not knowing the difference.
A HD ready TV is a TV with a HD screen but no built in way of receiving a HD signal, so it needs to be connected to an external HD source (such as a sky HD box, for example)
A HD TV has a HD screen and also has a built in way to receive HD TV (either a freeview HD tuner, or a freesat HD)
there is also 3 levels of HD, 720p, 1080i and 1080p.... generally 1080p (the best one) is referred to as full HD, the others are just HD.