LAN stands for Local Area Network. It's a group of computers which all belong to the same organization, and which are linked within a small geographic area using a network, and often the same technology (the most widespread being Ethernet).
Metropolitan Area Networks- connect multiple geographically nearby LANs to one another (over an area of up to a few dozen kilometres) at high speeds. Thus, a MAN lets two remote nodes communicate as if they were part of the same local area network.
A WAN (Wide Area Network or extended network) connects multiple LANs to one another over great geographic distances.