I have searched for this question, but I haven't found a good answer.
Even the Wikipedia Article on URIs does not explain it thoroughly. I thought it was the protocol for accessing a web page. e.g. HTTP/HTTPS/FTP, but the wiki article says otherwise.
Some URI schemes are not associated with any specific protocol (e.g. "file") and many others do not use the name of a protocol as their prefix (e.g. "news").
I know what part of the URL is the scheme. But my real question was what does it do?