On the next level are third-level domains. These domains are immediately to the left of a second-level domain. In the example.sitemadesimple.com example, "example" is a third-level domain. There can be fourth and fifth level domains and so on, with virtually no limitation. An example of a working domain with five levels is www.sos.state.oh.us. Each level is separated by a dot or period symbol between them.
Domains of third or higher level are also known as sub domains, though this term technically applies to a domain of any level, since even a top-level domain is a "subdomain" of the "root" domain (a "zeroth-level" domain that is designated by a dot alone).
Traditionally, the second level domain has been chosen based on the name of a company (i.e. microsoft.com). The third level was commonly used to designate a particular host server. Therefore, ftp.sitemadesimple.com might be an FTP server, www.sitemadesimple.com would be a World Wide Web Server, and mail.sitemadesimple.com could be an email server.