An encoder is that specky little program that manages to turns a (e.g.) 60 MB WAV file into that cool 5MB MP3 package.  The quality, on the whole, doesn't tend to deteriorate - it is all dependant on the ripper that you have used.  Should you decide that you'll be getting a program such as this, be sure you download the latest version of WinAMP, cause that'd be the DEcoder.

Honestly, there are just too many encoders on the market, in the shops and on the 'Net at the moment, so there's no real use in me listing any.

The range from Windows based to those operated by commands in a DOS shell.  Most are relatively slow, a few are pretty fast, but it all comes down to the personal preference and capabilities of the user (I recommend the use of a Windows program for those of you not so versed in the used of DOS).

If you find a decent one, drop me a line and I'll post up something sbout it and a link.