Asynchronous Transfer Mode (ATM) is a networking technology that supports voice, video, and data over a single network using fixed-size cells. It defines two types of interfaces: the User-to-Network Interface between user devices and switches, and the Network-to-Network Interface between switches. ATM provides advantages like support for multimedia services, scalability, and high bandwidth, but is less efficient than other technologies and had high initial costs that decreased over time as standards were still being developed.