Action nodes affect objects. For example, spawner can be used to spawn new object and destroyer can be used to destroy objects.
Trigger nodes are used for starting the execution flow. For example Button and Joystick belong to trigger category.
Nodes in the logic category can be used to manipulate values, or to change how logic is flowing.
Meta category holds nodes that don't fit into any other category.
Audio category nodes are used to generate or play sounds.
From the visual category you can find nodes that add or change visual elements in the scene.
Nodes in the Vector category manipulate vectors.
Physics nodes affect object physics.
Base nodes cover the basic properties of objects and their visual and physical properties.
Rotation nodes are more advanced and work with quaternions.
Multiplayer nodes are used to create logic for multiplayer games.