Top level domain
A TLD, or top level domain, is a domain which consists of only one part and is referenced directly from the root servers - such as com, net, org, or one of the country codes, like us or za. By convention, TLDs do not contain A records or MX records, so you cannot resolve one directly or send email to an address at it - there are no http://com webpages, or person@com email addresses. TLDs only contain NS records, to serve as a place to begin querying for the authoritative nameserver for a given URL.
There are also some "second level TLDs", such as Britain's co.uk and Australia's com.au. Although they are not, strictly speaking, "top level" domains, those countries reserve their use for NS purposes only, just like com, net, and org.