Like previously mentioned playlist format M3U, PLS is also a similar file format standard for creating playlists in media players, but in fact is a much more expressive format than the other counterpart.
A PLS file can retain song cache information, like its length unlike M3U. Being just a playlist file, it does not contain any music files in itself, but rather point to music files stored elsewhere. The PLS file format is often used to redirect to Internet radio streams, for example, if you want to play a radio stream from Shoutcast, you can copy the PLS file url of the station from the site and play it in a desktop media player like Winamp, Foobar, iTunes or VLC. You will have to use the Add Location Or URL option in the media players to add the stream and listen to it.
PLS files are not supported in Windows Media Player, unless you install third party codecs, like the K-Lite Code Pack.