Internet is the union of thousands of networks linked by a common set of technical protocols that allow users to each network to communicate or use the services located on any of the other networks.
The protocols are called TCP / IP or "suite" of TCP / IP protocols.
Internet began with ARPANET, but now includes networks such as the National Science Foundation Network (NSFNET), the Australian Academic and Research Network (AARNet), NASA Science Internet (NSI), the Swiss Academic and Research Network (SWITCH) and about 10 thousand other networks, large and small, commercial and research.
There are other major wide area networks that do not rely on TCP / IP and therefore are not considered part of the Internet. However, communication between them is possible through e-mail thanks to the gateway (access points), which act as "translators" between the different network protocols involved.