The Internet backbone is made up of many large networks which interconnect with each other. These large networks are known as
Network Service Providers or
NSPs. Some of the large NSPs are UUNet, CerfNet, IBM, BBN Planet, SprintNet, PSINet, as well as others. These networks
peer with each other to exchange packet traffic. Each NSP is required to connect to three
Network Access Points or
NAPs. At the NAPs, packet traffic may jump from one NSP's backbone to another NSP's backbone. NSPs also interconnect at
Metropolitan Area Exchanges or
MAEs. MAEs serve the same purpose as the NAPs but are privately owned. NAPs were the original Internet interconnect points. Both NAPs and MAEs are referred to as Internet Exchange Points or
IXs. NSPs also sell bandwidth to smaller networks, such as ISPs and smaller bandwidth providers.
Taken from
HERE
So, the internet 'back bone' is made up of a few networks with massive transit (bandwidth). NSP's are connected to one of these networks.
The simple (but not overly accurate) explanation is that ISP's get their 'bandwidth' from NSP's. NSP's get their bandwidth from the core internet itself.
It gets complicated because this is all done at Layer2 (packet layer) rather than Layer3 (IP) which we all recognise.