Like ATM, frame relay is a packet-based transport technology. Unlike ATM, it does not use cells of a fixed size. Still, traffic of different types can be mixed together over the same pipe, creating more bandwidth efficiency as compared to circuit-switched networks. It is also easier to create a fully meshed network using a frame relay service, because packets can be switched from one node to any other via a single connection to a service provider's network or to a network of frame relay switches.

Frame Relay Net Resources
Links to more detailed info and articles.