Deployment diagrams are used to visualize the physical architecture of a system. They are used to describe the hardware components, software components, and connections between them. Deployment diagrams can be used to show the physical layout of a system, including the nodes, components, and connections between them.
From Wikipedia
A deployment diagram "specifies constructs that can be used to define the execution architecture of systems and the assignment of software artifacts to system elements." To describe a web site, for example, a deployment diagram would show what hardware components ("nodes") exist, what software components ("artifacts") run on each node, and how the different pieces are connected.