Nodes are the basis of everything that Houdini does. When you create geometry, rig a character, and so on, Houdini is creating nodes behind the scenes. The nodes record the actions you took and the parameters you used. You can go "back in time" and change parameters and selections of existing nodes to edit the history of how your scene was created.
Channel nodes create, filter, and manipulate channel data.
Composite nodes create, filter, and manipulate image data.
Dynamics nodes set up the conditions and rules for dynamics simulations.
Object nodes represent objects in the scene, such as character parts, geometry objects, lights, cameras, and so on.
Render nodes either render the scene or set up render dependency networks.
Geometry nodes live inside Geo objects and generate geometry.
Containers for VOP networks.
VOP nodes let you define a program (such as a shader) by connecting nodes together. Houdini then compiles the node network into executable VEX code.
TOP nodes define a workflow where data is fed into the network, turned into "work items" and manipulated by different nodes. Many nodes represent external processes that can be run on the local machine or a server farm.