PrimitiveComponents are SceneComponents that contain or generate some sort of geometry, generally to be rendered or used as collision data. There are several subclasses for the various types of geometry, but the most common by far are the ShapeComponents (Capsule, Sphere, Box), StaticMeshComponent, and SkeletalMeshComponent. ShapeComponents generate geometry that is used for collision detection but are not rendered, while StaticMeshComponents and SkeletalMeshComponents contain pre-built geometry that is rendered, but can also be used for collision detection.