WWW is short for the World Wide Web. This 'www' is sometimes used as the prefix to a domain name such as www.domain.com due to a long standing practice. The 'www' prefix is in fact a subdomain of the root domain and is not necessary for functionality reasons. Many websites have adopted this practice of creating and using the 'www' subdomain to signify that this is the address where web pages can be accessed as opposed to ftp.domain.com where FTP access is available.
In most cases (unless special changes have been made), both the website, FTP and other services can be accessed through 'domain.com' and require no subdomain to be configured or specified when connecting. When registering a domain with 1&1, the creation of the 'www' subdomain is not necessary.