All domain names on the Internet actually end in a . (period) character -- that is, technically, Wikipedia is actually hosted on the domain "www.wikipedia.org." (try it.) This final dot is implied, and all modern DNS software does not actually require that the final dot be included when attempting to translate a domain name to an IP address. The final dot is called the root domain, and all other domains (i.e. .com, .org, .net, .uk, etc.) are contained within the root domain.
When a computer on the Internet wants to resolve a domain name, it works from right to left, asking each nameserver in turn about the element to its left. The root nameservers (which have responsibility for the . domain) know about which servers are responsible for the top-level domains. Each top-level domain (such as .org) has its own set of servers, which in turn delegate to the nameservers responsible for individual domain names (such as wikipedia), which in turn answer queries for IP addresses of subdomains (such as www).
In practice, most of this information doesn't change very often and gets cached, and DNS lookups to the root nameservers are relatively rare.
There are currently 13 root name servers, with names in the form ?.ROOT-SERVERS.NET where ? runs from A to M, namely:
|B||ns1.isi.edu||ISI||Marina Del Rey, CA|
|D||terp.umd.edu||University of Maryland||College Park, MD|
|E||ns.nasa.gov||NASA||Mountain View, CA|
|F||ns.isc.org||ISC||Palo Alto, CA|
|G||ns.nic.ddn.mil||U.S. DoD NIC||Vienna, VA|
|H||aos.arl.army.mil||U.S. Army Research Lab||Aberdeen, MD|
Older servers had their own name before the policy of using similar names was established.
No more names can be used because of protocol limitations, but the C, F, I, J and K servers exist in multiple locations on different continents.
There are quite a few alternate namespace systems with their own set of root nameservers that exist in opposition to the mainstream nameservers. The first, AlterNIC, generated a substantial amount of press. See Alternate DNS root for more information.