When you use the name of a domain, your computer looks up the corresponding number to address the packets, and the computer on the other end can look up the number to find out the name that they came from. The only valid characters for a domain name are letters, numbers and a hyphen. Domain names cannot contain spaces or begin or end with a hyphen. A username is a name for logging into a system. A username is always accompanied by a password to grant some form of security.
Server is for outgoing mail.
POP3 Server is for incoming mail. File Transfer Protocol. FTP is the most commonly used way to transfer files through the Internet. Most people use FTP to upload their websites. Hypertext Transfer Protocol.