The Internet (note the capital 'I'; just as there are 'white houses', but only one 'White House', there are many 'internets', but only one 'Internet') is the world's dominant information network.
Its most important technical consituent is the Internet Protocol, a protocol which offers direct datagram carriage across the entire network, providing an unreliable service which makes no guarantees that packets will not be damaged, delayed, duplicated or re-ordered. This lack of any delivery guarantees makes the job, and implementation, of the packet switches that form the lowest layer of the networking infrastructure (along with the physical networks that connect them) much simpler.
This means that it is the job of the protocols above the internetworking layer to ensure reliable data carriage on an end to end basis; these do so using sequence numbers, timeouts, and retransmission.
It is to some degree a direct descendant of the ground-breaking ARPANET, but only in the sense that its early dominant application protocols (TELNET, FTP, and email) were direct clones of those of the ARPANET; and that the technical community which created it was an overlap/descendant of the one that produced the ARPANET.
At its bottom layers, the Internet is to some degree sui generis, although it was heavily influenced by the ground-breaking CYCLADES network.