The National Science Foundation (NSF) created the first high-speed backbone in 1986. Called NSFNET, it was a T1 line that connected 170 smaller networks together and operated at 1.5 Mbps (million bits per second). IBM, MCI and Merit worked with NSF to create the backbone and developed a T3 (45 Mbps) backbone the following year.
Backbones are internet connections that allow vastly more traffic than the connection from your home to the central office around the corner. In the early days of the internet, only the largest telecommunications companies had the ability to handle that sort of bandwidth.
Today more companies operate their own high-capacity backbones, and all of them interconnect at various IXPs around the world. In this way, everyone on the internet, no matter where they are and what provider they use, can talk to everyone else on the planet. The entire internet is a gigantic, sprawling agreement between people to intercommunicate freely.