Nuke and nodel modelling
There are often many different node types participating in the node graph. For example in the Nuke Manual[dead link] they list hundreds of nodes. Each node type performs one specific task. For example Nuke's Merge node produces an output image in which a number of input images have been layered. By connecting many different node types together complex image effects can be produced.
The node graph architecture often allows grouping of nodes inside other group nodes. This hides complexity inside of the group nodes, and limits their coupling with other nodes outside the group. This leads to a hierarchy where smaller graphs are embedded in group nodes. In Nuke the group node is simply called the Group node.