In UML modeling, component instances are model elements that represent actual entities in a system.
You typically use component instances in deployment diagrams to represent implementation units that exist at run time; however, you can also use them in component diagrams.
The name of a component instance consists of an underlined concatenation of the instance name, a colon (:), and the component name; for example, Shopper1:Cart.
As the following figure illustrates, a component instance is displayed in the diagram editor as a rectangle that contains the name and stereotype.
You are developing an e-commerce application that distributes components among the customer's Web browser, a public Web server, and a private data server. To specify on which device a specific component (for example, the Cart component) runs, you can create a deployment diagram with three node instances: one for the computer running the Web browser, another for the Web server, and a third for the data server. You can then create a deploy relationship between the Cart component instance and the Web server node instance to indicate that the component instance is run on the Web server.