Introduction to primitive collider shapes

Primitive colliders are the most computationally efficient type of colliderAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
See in Glossary in Unity. They are called “primitive” because they are defined by simple geometric shapes such as boxes, spheres, and capsules. They match the Primitive Objects, which are built-in GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary shapes.

There are three primitive collider shapes in Unity:

• Box colliderA cube-shaped collider component that handles collisions for GameObjects like dice and ice cubes. More info
  See in Glossary: A rectangular box-shaped collider that is suitable for most rectangular objects.
• Sphere colliderA sphere-shaped collider component that handles collisions for GameObjects like balls or other things that can be roughly approximated as a sphere for the purposes of physics. More info
  See in Glossary: A spherical collider that is suitable for most circular objects.
• Capsule colliderA capsule-shaped collider component that handles collisions for GameObjects like barrels and character limbs. More info
  See in Glossary: A cylindrical collider that is suitable for objects that have a cylindrical shape, or for rounding out the collisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
  See in Glossary edges on sharp corners. Capsule colliders are also useful for player and non-player characters.

Primitive colliders are efficient, but they have limitations. For example, you cannot change or deform their shape, only their scale. Unlike MeshThe main graphics primitive of Unity. Meshes make up a large part of your 3D worlds. Unity supports triangulated or Quadrangulated polygon meshes. Nurbs, Nurms, Subdiv surfaces must be converted to polygons. More info
See in Glossary colliders, they are not made up of triangles; their shape is fixed (note that the Physics Debug visualization does indicate a triangle-based mesh on primitive colliders, but these are for visualization purposes only and do not reflect the collider’s construction).

Primitive colliders are usually not the best option for complex shapes, wheels, or TerrainThe landscape in your scene. A Terrain GameObject adds a large flat plane to your scene and you can use the Terrain’s Inspector window to create a detailed landscape. More info
See in Glossary collision. Unity has specific colliders available for more complex collider shapes (see Mesh collidersA free-form collider component which accepts a mesh reference to define its collision surface shape. More info
See in Glossary, Terrain collidersA terrain-shaped collider component that handles collisions for collision surface with the same shape as the Terrain object it is attached to. More info
See in Glossary, and Wheel collidersA special collider for grounded vehicles. It has built-in collision detection, wheel physics, and a slip-based tire friction model. It can be used for objects other than wheels, but it is specifically designed for vehicles with wheels. More info
See in Glossary). However, if you need a collider shape that fits a complex shape, but does not need to be too accurate, you can use primitive colliders to create a Compound collider. A Compound collider is a collection of primitive colliders in an arrangement that fits the collider shape you need.

In summary, primitive colliders are an efficient but sometimes inaccurate way to add collision detectionAn automatic process performed by Unity which determines whether a moving GameObject with a Rigidbody and collider component has come into contact with any other colliders. More info
See in Glossary to objects in Unity. They are suitable for primitive-shaped GameObjects with regular shapes. However, for more complex objects with irregular shapes, or for more accurate collision detection, you should use a more complex collider shape.