MP3s are compressed music files that are one-tenth the size of regular audio files. As a normal audio file requires about 40 megabytes to download, and MP3 requires four. Although this is still a substantial number, it is much more manageable for the average computer. Thus began the MP3 downloading, sharing and trading craze. There are a variety of options when it comes to playing your MP3s. Media Player comes with Windows, QuickTime 4 comes with Macintosh, and Win amp is the industry standard product.