Hey Sue, lots of great advice here, but every digital encoding has a sample rate. The higher the sample rate the larger the file but the higher the quality.
Think bananas: You can devour it in four bites and it will be soon be gone but the full flavour hit will be strong. If you take forty bites the flavour may not be as strong but it will last much longer. Now how many bites do you need to compromise between getting the best of flavour with the longest duration. That in a nutshell is compression and mp3's can be compressed to the most bites while wav's take up the total time to play (eating) them. The sample rate is critical but bear in mind that you can't shove the toothpaste back into the tube. You can convert from wav to mp3 with a small loss of quality but if you go from mp3 to wav you will not restore it back. What is lost becomes lost forever. Some players will not play mp3's but most of the newer ones will. A CD will allow either 700 mb of storage or 80 minutes of play. (either/or) With a wav file you are limited to the 80 minutes (and only as many songs you can play in that time) With an mp3 you can play as many songs as the data limits will allowl, but you suffer degraded quality.