Broker: A company in the business of buying and selling domain names. Domain name brokers usually act as facilitators between buyers and sellers, but very rarely buy for their own account. Note that many companies call themselves "brokers" because it basically sounds good, though all they really do is list domain names for sale on their site, with contact information...
Country code top level domain (ccTLD): A generally two-letter top domain that stands for a specific country. Examples include .fr for France, .de for Germany, .br for Brazil and so on. Most CTLDs are controlled by an authority in the relevant country, though the rights to a few CTLDs have been sold to private parties. One well-known case is the ".tv" CTLD, which is operated by Dot.TV - the ".tv" designation originally stood for "Tuvalu" but that country ceded all rights over the .tv designation for at least 10 years, in exchange for a large sum of money.
Dropped domain: A previously registered domain name whose registration was allowed to lapse by the original owner, who refused to pay the renewal fee. The domain name returned to the "available" pool of domain names.
Expired domain: A domain name whose renewal date has passed, but which has not yet been dropped from the domain name system. Many people track "good" expired domains in the hope that they can be the one to register them when they drop.
Extension: The final portion of a domain name beyond the right-most "." For instance, the ".com" extension; the ".fr" extension.
ICANN: Non-profit organization in charge of overseeing domain name disputes and maintaining overall control of the domain name system.
Name server: A machine that contains a local record mapping domain names and IP addresses. Domain names reference two name servers: a primary name server and a secondary name server. The secondary is used if the primary is inaccessible for any reason.
Nominet UK: Non-profit limted company in charge of overseeing domain name disputes and maintaining overall control of domains ending in .uk.
"On Hold": A domain name is said to be on hold when its record is locked in the domain Registry. Domain names that are on hold cannot be used to point to a given website. Domain names may be put on hold during a dispute between two parties over the rightful ownership of the relevant name, so that neither party can make active use of the name for the duration of the dispute. Domain names may also be put on hold shortly before they drop, if they have been allowed to expire.
Registrar: A company in the business of registering domain names. An accredited registrar is permitted to update the domain name database (maintained by the Registry) directly. Other registrars work through partnerships with accredited registrars. Registrars pay a minimum fee per domain name to the Registry for the right to record the ownership of the domain name.
Registry: A non-profit company in charge of maintaining the master database of all domain names, including details of ownership, expiry dates etc.
Renewal date: The date on which the periodic maintenance fee paid for a given domain name runs out. On or before this date, the domain name owner must pay another 1-10+ years of domain name maintenance fees in order to keep the rights to the domain name (gTLD domains only). If the renewal date is allowed to pass without payment, the domain name becomes expired and will eventually drop.
Second-level domain: The portion of the domain name before the name.extension. For instance, www is a very common second level domain of most domain names. Second level domains can be used to "stretch" a domain name further by allowing one domain name to serve for multiple pages or sites (e.g. mail.domain.com and help.domain.com both start off using the domain.com domain, but can refer to different sites)
Top-level domain: The end portion of the domain name including the final "." - for instance .com, .net or .org. Some country domains have a double-barreled top-level domain, such as .co.uk or .or.jp.
Transfer (I): The process of moving authority over a domain name from one domain registrar to another, usually to gain a price advantage or take advantage of the more lenient Terms & Conditions offered by the destination registrar. Each registrar has its own rules and systems for transferring domain names, and these are not always compatible with each other.
Transfer (II): The sale of a domain name is sometimes known as a "domain transfer" since in the case of a sale what really happens is a transfer of rights in the given domain name, which is reflected by a change in the ownership details recorded in the Registry. This change can be effected by the registrar that registered the domain name, or can be carried out via a transfer (I).
Transfer agreement: The documentation required to formalize or authorize a domain transfer from one party to another (i.e. in the case of a change in domain name ownership). Often, such a change in ownership requires a notarized transfer agreement completed by both parties to the transaction.