Some MP3 players use solid state memory to store the music. Solid state memory comes in the form of Flash memory or onboard ram. MP3 players using solid state memory tend to be smaller in size than hard disk based players but lower in storage capacity. The storage size of solid state MP3 players range in size from 32MB up to 8GB.
The advantage of using solid state memory is that it has no moving parts, this means that the music can be played whilst jogging or exercising without the music skipping.
The down side to solid state memory is that it is more expensive per megabyte of storage compared to hard disk based player.