【Primitive Objects】
Unity can work with 3D models of any shape that can be created with modelling software. However, there are also a number of primitive object types that can be created directly within Unity, namely the Cube, Sphere, Capsule, Cylinder, Plane and Quad. These objects are often useful in their own right (a plane is commonly used as a flat ground surface, for example) but they also offer a quick way to create placeholders and prototypes for testing purposes. Any of the primitives can be added to the scene using the appropriate item on the menu.
原文：http://www.cnblogs.com/tekkaman/p/3536879.html