DNS or Domain Name System is a service most commonly used to translate Domain Names (URL's, websites, call them what you will) into IP Addresses. Realistically, IP Addresses are the true identifiers of how to locate things/places on the Internet, however DNS makes it much simpler to "surf" by only having to reference Domain Names that are much easier to remember. DNS functions in a hierarchical structure
Rackspace uses an implementation strategy called AnyCast with our Authoritative DNS. This technology allows us to announce the same DNS IP space from 3 different (or as many as desired) datacenters via the BGP protocol we run with our Internet Service Providers. The purpose of this is multi-fold in that it not only allows us to have active/active DR type redundancy, but it also allows us to serve customer requests from the closest DNS infrastructure possible, which typically means the best response time too. Additionally, it allows for the ease of maintenance, upgrades, and expansion our our DNS infrastructure with no customer impact.
This is the same kind of underlying technology that the Root Nameservers of the Internet run off of, and many other companies that specialize in DNS as a service.