*.wav's are pretty much universal, but are also HUGE!!! (I recently cleaned out my computer, converting all my wav's to mp3's and got over a gig more free space)
*.mp3's are universal for computers and can play on some CD players
*.wma's can be played on any CD player if you use the function in a CD burning program to make an Audio CD, but they can only be played with certain programs on a computer. also, they are slightly smaller than mp3's
*.m4p's can only be played on iTunes and iPods, but are almost as small mp3's
there are probably more formats out there, but I can remember them, so you shouldn't use them unless you have some program that thrives with them.
RealPlayer or not (and yes, RealPlayer is awful), here are a couple things to consider:
If you are making a data CD of MP3s to play in a CD player that can read "MP3 CDs," I would say use at a minimum 192kbps VBR (variable bit rate) encoding. MP3s themselves are pretty terrible, and destroy sound quality. But at 192 and above, they sound... well, reasonable. If you have the space, go higher (256 or 320). Oh, and sample rate should always be 44.1 kHz.
If you are making an audio CD, to play in a normal CD player, stick with WAV or AIFF. WAV and AIFF, though large, are uncompressed and equal to the original CD quality. If you take a CD, rip it into your computer as MP3s, and then burn an audio CD from your MP3s, you are doing the equivalent of taking a CD, recording it onto a cassette tape, then burning a second CD from the cassette tape. What you end up with on the burned CD is definitely of lower sound quality than the original CD.