aptX is an audio-coding algorithm that allows you to transfer music wirelessly with CD quality.
Now for aptX HD. This is essentially an updated, beefed-up aptX with the ability to transfer music in a way that allows better sound quality.
It was released in reaction to the increasing popularity of hi-res audio, and supports audio at 24-bit/48kHz. Compression remains at a ratio of 4:1, with a bitrate of 576kbps.