I just wrote about the TLD, so this naturally follows:
A SLD includes the TLD name and further identifies the owning organization of a URL.
Second-level domains can be divided into further levels. These subdomains sometimes represent different computers within an organization, but are many times the same machine with different aliases.
TLD = .com
SLD = giantgeek.com
Subdomains = www.gianteek.com & mail.giantgeek.com
NOTE: Refer to HTTP/1.1 for details on how IP addresses, routing and webservers are impacted by this.
Interesting enough, some one character SLD’s do exist, x.com for example.