ATM is a WAN standard. ATM is a network that has been standardized by IEEE to be used in creating WANs. On the physical level, it uses a gigantic network of high capacity (bandwidth) optical fiber cables to connect LANs of different geographical areas.
ATM works on the virtual circuit switching technology. It uses VCIs (Virtual circuit identifiers) to address to the parts of Network. The size of packets in ATM networks is fixed. These fix sized packets are called Cells and hence ATM is sometimes referred to as Cell Switching Network.
ATM is a very versatile network. It can transfer voice data along with text and graphics. In other words, ATM can carry both the circuit and packet data along its physical networking structure.