HDTV-ready means that the tv can display hd quality television but lacks a tuner for HD. So you need a separate tuner in order to receive and view HD digital tv. HDTV can display hd and has a hd tuner, you just need to plug the antenna.
A HDTV is literally a television that can show high definition picture quality. A HDTV-ready television can also work with a HDTV signal but its picture quality is lower.
HDTV Ready televisions only pick up the digital signal if an exterior digital tuner is purchased. HDTV's have the digital tuner already installed and will pick up all digital stations.