For example I meet this url type: http://username:token@example.com/protected/files
.
I searched on the web for this but I don't find what I expected.
Wikipedia explains the syntax of a URI quite well:
scheme:[//[user[:password]@]host[:port]][/path][?query][#fragment]
Your username:token@example.com/protected/files
may look like and URL, but if fact it is not, because it does not include the protocol to access the data. It is an uri.
Browsers (I suppose you refer to web browsers) do work with URL's, which is a subtype of URI that includes the protocol. Please notice web browsers dont work with all existing protocols, only with some of them (http, https, ftp, file...) username
is not a protocol.