There are many audio formats on the web and each one of them has their cons and pros. There are too many of them for me to look at all of them, so I will only look at MP3, Ogg Vorbis, FLAC, WMA, and WAV.
MP3 is considered the standard audio format, because it is the most widely known and used. It is well supported in Linux, Windows, Mac, iPod, and can even be played in Windows Media Player and in iTunes. It has a small size because it cuts of the parts that your ears cannot hear, which makes the music sound exactly as we would expect. It is considered a lossy format, and will lose data in data compression.
Ogg Vorbis is an open source audio format and is compared to MP3. It is supported by default in Linux, but not in Windows Media Player and iTunes. It is a lossy format like MP3, but the quality is better. Ogg at 92 kps will sound very good, but MP3 needs to be at 320 kps to achieve the same quality.
FLAC is a loseless format and brings you closest to the CD quality. It remains the standard for top of the line quality, but you pay a price for it. A single track can be up to 100MB. It is supported by few portable players.
WMA is an unusual format that is produced and nearly solely supported by Microsoft. It is a very hard to find format, but is often found in music stores that use DRM. It was made to compete with MP3 and thus it has a decent file size. It is widely supported by iPods, and iTunes.
WAV has a large file size when it is uncompressed. It is a very hard to find format and nobody really uses this format. It is used on your Windows system for the system sounds. As time goes on, this format keeps on dying.
I personally use the OGG format because it is comparable to MP3 and is open source. If your a normal user and just wants something that works I would recommend MP3, because it is decent and widely supported. If your a quality freak and want the best quality you can get I would suggest the FLAC format.