The Internet backbone is a number of high speed fiber optic cables which cross the world. Within the US there are numerous highspeed cables owned by major communications companies such as GTE, UUnet, AT&T, MCI, Sprint, and Verio. This is but a few and there are many others. OC3 and OC12 and OC48 lines are typical. The The table below shows the speed each type of cable is capable of. Typically the OC12 and OC48 lines are used to cross the country intersecting at major Network Access Points. The T3 and OC3 lines generally connect data centers to these backbones.

Each of the major Internet Communications carriers of which there are well over 50, own their own Internet Backbone. A good place to view the maps of each of the carriers is at Mapnet. Here you can view the world and display selectively each of the carriers backbone and superimpose others to the point where you can not see the US as it is covered by the lines.

All of these individual backbones at one point or another interconnect with each other at Network Access Points (NAPs) via Internet Routers. Presently, as far as I could find out, there are 12 NAPs in the continental US. Many routers are stationed in each of the NAPs serving each of the individual backbones. These Routers act as traffic directors looking at the destination of each packet of data and directing it to the proper backbone where it is then routed to it's final destination. The good thing about having multiple backbones interconnected together is redundancy and speed. As the Internet becomes congested, data normally routed on one backbone could be transfered/shared by another backbone and if one backbone becomes inoperable, all the data could be rerouted to another backbone thus keeping the Internet alive.