Because of vital importance, high load and especially because of need to tolerate any possibly DDoS attack, internet have many root servers.
All root servers store the same DNS database.
A through M is total of 13 nameservers, but to be exact each this "nameserver" is not actually a server, but it is an IP address.
For most of root servers, that IP address corresponds not to single machine, but it is a so-called anycast address - address which can be routed to any of several locations around the globe. Thus when you query, say f.root-servers.net, your query may be answered by a server located at any of this anycast sites (usually the one nearest to your).
"A" root server is also anycasted.
Different root servers (A, F etc.) operated by different organisations, thus lowering possibility of some human mistake to take down the whole internet.
There's a map of root server locations on root-servers.org.