An mp3 player is a device that plays music. Some more advanced mp3 players can play videos and pictures, show ebooks and listen to the radio. Some expensive mp3 player, such as Apple's iPod touch and Microsoft's Zune HD, can browse the internet, run applications, and take pictures. Mp3 players range in price from five dollars to three hundred dollars.
Some popular mp3 player manufacturers are Sandisk, Apple, Microsoft, Visual Land, Polaroid, Sony, and many others. There have been some efforts to replace the firmware on popular mp3 players to increase usability and functionality. For more information, see http://en.wikipedia.org/wiki/Portable_media_player.