Collider 2D components define the shape of a 2D 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 for the purposes of physical collisionsA 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. A 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, which is invisible, need not be the exact same shape as the GameObject’s 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; in fact, a rough approximation is often more efficient and indistinguishable in gameplay.
Colliders for 2D GameObjects all have names ending “2D”. A Collider that does not have “2D” in the name is intended for use on a 3D GameObject. Note that you can’t mix 3D GameObjects and 2D Colliders, or 2D GameObjects and 3D Colliders.
On the Rigidbody 2D component, tick the Use Auto Mass checkbox to automatically set the Rigidbody 2D’s mass to the same value as the Collider 2D’s mass. This is particularly useful in conjunction with the Buoyancy Effector 2D.