Downloading music through the internet, rather than buying physical media like CDs and vinyl records, is becoming increasingly popular with the advent of services like ITunes and Spotify.
These services provide instant access to vast catalogues of music combined with the convenience of the music being delivered in a format that can be listed to on numerous devices such as mp3 players, smartphones and computers. In addition, the music filed can be written to usb drives or burn to CD/DVD so that they can be listed to on an even wider range of devices, like home audio systems and in the car.
There are essentially two main models for music downloads at present:
1) Purchase individual tracks/albums - eg iTunes
This is the traditional approach, where consumers purchase the specific albums and tracks that they wish to listen to. Most services allows individual album tracks to be purchased, rather than just complete albums, although there is usually a considerable saving if purchased as a complete album rather than as a series of album tracks.
2) Pay a monthly fee to access all the music held by the service in question - eg Spotify
This is where a regular monthly payment is made for access to a vast array of music titles. The listener does not own the music, they are just paying for the ability to listed to it. Some of these services allow the music to be stored offline and listened to through specific smartphone apps and computer programs.
The quality of the downloaded music will vary according to the type and bitrate of the sound files. Most services deliver the music in a compressed form, which helps to reduce the size of the files at the expense of absolute accuracy to the original recording. For most purposes, this isn't really noticeable due to the playback devices - but audiophiles often prefer to purchase a CD and make their own lossless digital copy of this to retain ever last nuance of the recording.