strictly speaking an HD ready TV is a TV capable of showing HD if it's connected to a source that can provide HD (i.e. a blue-ray player or a sky HD box)
A HD TV will have a built in source so it can show HD without it needing an extra box (i.e. the TV will have a built in freesat receiver)
the one thing to make sure is you get a full 1080p HD set (lower quality HD sets are either 1080i or 720p)