NETWORK: Two or more computers which are linked together and which share resources like data, programs and peripherals (like printers) are networked.

Types of Networks
LAN (Local Area Network)
Covers a short distance (usually less than 1 kilometer [<> 1 km)]
Usually spans several locations (even world wide)
Usually has a larger number of users (100's or even 1000's)
Implemented as a client-server model Peer to Peer Networks
All computers on the network have the potential to share resources that they have control over
NOT a client-server model (in the strict sense of that concept)
usually confined to a small area (an office or a lab)
Usually has a small number of users (around 2 to 25 users)
considered to be less secure than a LAN or a WAN
see above for more details NOTE: you may have hybrids (combinations) of LANs, WANs and Peer-to-Peer networks
Intranets vs The Internet
The Internet is a collection thousands of Wide-Area and Local-Area networks with servers (http, ftp, chat, ...) on them. The Internet uses TCP/IP as the primary networking protocol to communicate with all these machines.
Intranets are local networks that do not have access to the outside world (they are self-contained). They are like the Internet but they are private.