Battery life: this varies a lot. Small flash memory players without LCD screens tend to last the longest. The actual figure in hours will be quoted by the manufacturer.
Capacity: the amount of memory dictates how many songs you can fit onto a device, and this figure is always prominently marked on the packaging in MB (megabytes) or GB (gigabytes).
Resolution: if you want a device that can also show digital images or video then screen quality is important. This is given as the number of pixels along the horizontal side, times the number along the vertical, for example 480 x 320 - great quality.
Software: MP3 players include computer software, so you can organise and load music onto the player. A programme called iTunes is bundled with iPods, and it works with both Mac and PCs. Other software might only be compatible with PCs.