File

A file is a group of data which, usually, has some sense.

Examples of files are text files, music files, video files, etc.

Each file has it's own encoding. Applications exist to read this files, and, knowing it's encoding methid, decode them and make out of this files something usefull.

aMule, for example, knows the encoding of it's configuration files, which allow it to upload and download (let's just say, share) other files.

The target of any P2P client is exactly to share files.