Node refers to a scene element or sub-element. A node is a generic container that can hold different types of information, depending upon the node type (and there are effectively an infinite amount of node types that an application or development engineer can create.
Nodes can hold transform information, geometric information, shading information and so on. In addition to being containers of information, nodes can also be operators, and can perform math functions, for example. In some cases, a node can be a simple add node, where you can have one node add to another, or add a value to the value already present in the operator node. You can also have more complex nodes that can solve various levels of complex problems—in reality, these complex nodes are typically made of simpler nodes put together to form a higher level operation.
When nodes are chained together, the result is called a “node tree.” Node trees can be simple to very complex. In order to create a node chain or node tree, you must pipe the output of one node to the input of another. The output of that node would be dependent upon the goal of that branch of the node tree.
Nodes may have multiple inputs and outputs. Depending upon the actual contents of the node, its possible that the numbers of inputs and outputs of the node is actually variable.
Because of all of this versatility, nodes are commonly used for digital compositing and the internal datastructures of 3D content creation applications. In some cases, the nodes may actually be disguised from the user, but its far more powerful to allow the user to work with the actual nodes themselves.
Xpresso is the node editor in Cinema 4D, and Thinking Particles also relies on nodes to build rules for the particle systems and interactions. From the online documentation: “Nodes are the primary building blocks of expressions and are designed to carry out the most diverse of tasks, from reporting an object’s current position to processing math operations. Depending on the node’s type, you can add various inputs and outputs to the node called ports. As with XGroups, you add these ports using the inputs menu and outputs menu (the blue and red squares respectively in the node’s title bar).”