An MP3 is a digital audio file compressed using a standard defined by the Motion Pictures Experts Group (MPEG). Officially known as "MPEG-1, Layer 3", MP3s can use as little as 10% of the storage required by the CD format. An MP3 player is a device for playing MP3 files.

MP3 players come in many sizes and capacities. The smallest MP3 players use a type of memory known as flash; unlike conventional memory such as that found in a desktop computer, flash does not lose stored information when the power is turned off. Flash is very compact and uses very little power, allowing for extremely small designs with long battery life.

The limitation of flash-based MP3 players is that they do not have a great deal of storage capacity. For larger storage needs, hard disk-based MP3 players were developed. These can have storage capacities in the tens of thousands of songs, but sacrifice the ultra compactness of flash-based players. Hard disk-based MP3 players are also generally more expensive than flash-based players.

While not invented by Apple, it was Apple's line of iPod MP3 players that popularized the devices. With their distinctive white headphones and Lucite and chrome styling, coupled with extreme ease of use, iPods soon dominated the portable MP3 player market.

With the rise in popularity of MP3s as a format for storing music, a number of non-portable MP3 players appeared for the home market. Sometimes referred to as "digital hubs" or "media centers", these devices are a cross between a stereo component and a computer. With network connections, often wireless, they are able to access MP3s and other digital content stored on computers on the home network. With a full array of standard audio/visual connections, they are able to integrate with home entertainment systems. These devices come with remote controls and may use a television as a display screen for menus and playback information.

A major advantage of these devices when used as MP3 players is that more than one device can access your MP3 files at the same time. All the music can be stored in one place, and the various MP3 players in the home can treat the MP3 collection like a library with unlimited copies of every book.