National Science Foundation Network (NSFNet) was a major part of early 1990s Internet backbone.
Organisations connecting to the Internet in the early 1990s had to sign a usage agreement directly with NSFnet to gain access to large parts of the Public Internet, regardless of what Internet Service Provider they purchased Internet access from.
From 1987 to 1995 the NSFNET was operated on behalf of the NSF by Merit Network, Inc. , a non-profit corporation governed by public Universities.
On April 30, 1995, the NSFNET Backbone Service was successfully transitioned to a new architecture, where traffic is exchanged at interconnection points called NAPs (Network access points.)
NSFNet is unrelated to NFSNET, a distributed computing effort to factor large numbers. Also NSFNet has nothing to do with Network File System (NFS), a TCP/IP file sharing protocol like SMB.