FLAC = Free Lossless Audio Codec.
It was invented by a smart American engineer who hated the Lossy compression codecs like MP3, AAC, Nero etc. So he invented one of his own that when you "Expand" it back to full size doesn't suffer any loss of integrity.
WAV files = 1411 KBS
FLAC files = 900 KBS depending on how small you want to compress them.
MP3 = 320 KBS
AAC = 256 KBS
My Sansa plays anything, FLAC, WAV, AAC, MP3, Ogg Vorbis so if you can update the firmware on your MP3 player you might be able to load a FLAC app and play them Harkat. I know many i-pod owners use many of the free available apps to get around Apples insane closed system and can play anything they want.
Google Foobar 2000 and download it if you like, it is a great little freeware app. and you can play or convert FLAC to WAV with it on your computer at least.