DirectShow FilterPack is a lite-weight package of filters (decoders) which will enable you to play many different kinds of encoded music and video. By default you can already play MP3 music, because of the Fraunhofer MP3 Decoder shipped with Windows.
But unlike MP3 files, Windows doesn't know how to handle Vorbis Audio for instance. With the OGG Splitter inside this package (plus FFDShow) Windows can handle Vorbis Audio.
This package was designed to work with FFDShow. FFDShow is a very powerfull DirectShow filter for audio and video decompression, like DivX/XviD and Vorbis/AAC for instance, but can do much, much more. This way I can keep my package as small as possible, unlike certain codecpacks who install more than one filter for the same thing.