DSD (Direct Stream Digital) is a Sony/Philips audio format consisting of uncompressed audio at an extremely high sample rate. Related to Super AudioCD format. Its files are really really large, but audiophiles claim they sound much better than the lower-sample-rate, compressed files of other formats. Some question this, on the grounds of the quality differences being at a level well beyond the ability of the human ear to discern.
There is a "DSD Disc" format for use on discs, as well as a DSD Interchange File Format (DSDIFF) for downloads. There are apparently two variant download formats, with the extensions .dsf and .dff, where the main difference is that the .dsf format can hold metadata in addition to audio data.