A network is more than one computer that can communicate directly to others.
LAN stands for Local Area Network - a network that is usually small an on one site only.
WAN stands for Wide Area Network - a network that can be vast, with computers at many geographical locations.
Computers on networks are known as terminals.
Servers are powerful computers that manage computers on a network. Sometimes they do a specific job: print, file or web serving.
There are good reasons for having networks, but some pitfalls. e.g. cheap communication but easy to spread viruses.
Networks can be put together in a number of ways, called topologies. e.g. star, bus and ring networks.