Following the scheme in a URL, there can be a possible user name and password field embedded. The use of this syntax is usually frowned upon these days since you easily leak this information in scripts or otherwise. For example, listing the directory of an FTP server using a given name and password:
The presence of user name and password in the URL is completely optional. curl also allows that information to be provide with normal command-line options, outside of the URL.
If you want a non-ASCII letter or maybe a : or @ as part of the user name and/or password, remember to "URL-encode" that letter: write it as %HH where HH is the hexadecimal byte value. : is %3a and @ is %40.