Asynchronous Transfer Mode is a set of protocols used for computer networks. It is mostly used for Wide Area Networks (WAN). It splits the data, and encodes it into packets of a fixed size. Each packet can be routed differently, along virtual paths set up in the network.
Standards like Ethernet (which is used for LANs) usually are quite simple. When there is little traffic on the network they work well. However, when there is a lot of traffic it is difficult to predict how an Ethernet network will behave. For this reason, Ethernet cannot give any service guarantees. It is not possible to say that an Ethernet packet will take a maximum delay, but after that time it is guaranteed to arrive.