The PSF format was publicly documented by Neill Corlett in 2003, who also wrote a Winamp plug-in named "Highly Experimental" that plays PSF1 and PSF2 files.
Generally, PSF files contain a number of samples and a music sequencer player program.
This takes far less space than an equivalent streamed format of the same music (WAV, MP3) while still sounding high fidelity.
Several PSF sub-formats also have a miniPSF/PSFlib capability, wherein data used by multiple tracks is stored only once in an accompanying PSFlib file.
GSF players emulate the files as sound-only Game Boy Advance ROMs, and as such can be processor intensive when compared to mainstream audio formats.