HDCD is an encoding process that stores 4 bits of data in the 16 bit audio file. It can be decoded by a Pacific Microsystems HDCD digital filter giving you 20 bit resolution at playback. The filter is an excellent filter for both CD and HDCD. Most HDCD players that I have seen and heard are of good quality.
HDCD encoded discs will play on all CD players. The Pacific Microsystem digital filter can work with any CD (the chip can also be found in some DVD players).