The distributed virtual computer (DVC) is a computing environment, which simplifies the development and execution of distributed applications on computational Grids. DVC provides a simple set of abstractions to simplify application management of naming, security, communication, and resource, easing use of highly dynamic and heterogeneous resource environments. These abstractions enable complex collections of Grid resources to be used in a fashion similar to private user or workgroup resources. The DVC model is attractive for lambda-Grids with circuit-switched optical networks, providing a structure for exploiting unique communication and security properties. Examples of DVC include virtual clusters and virtual heterogeneous resource collections. We introduce the concept of a DVC, its system structure and mechanisms. We discuss the potential benefits of DVC for application programmers.