2. What is different between computer networks and internetwork?

The term "Internet" is an abbreviation of the term "Internetwork." An Internetwork is a network of networks. Just like Interstate highways connect states together, the "Internetwork" or "Internet" connects networks together.

internetwork packet exchange

A network of networks which consists of two or more physical networks connected by routers. Unlike a WAN and internetwork may reside in only a single location. Because it contains too many computers or spans too much distance an internetwork cannot fit within the scope of a single LAN.

Internetwork Packet Exchange. It was a protocol for Novell NetWare, but TCP/IP is so prevalent now, even IPX sites typically utilize it.