Collider 2D

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.

The Collider 2D types that can be used with RigidbodyA component that allows a GameObject to be affected by simulated gravity and other forces. More info
See in Glossary 2D are:

• Circle Collider 2D for circular collision areas.
• Box Collider 2D for square and rectangle collision areas.
• Polygon Collider 2D for freeform collision areas.
• Edge Collider 2D for freeform collision areas and areas which aren’t completely enclosed (such as rounded convex corners).
• Capsule Collider 2D for circular or lozenge-shaped collision areas.
• Composite Collider 2D for merging Box ColliderA cube-shaped collider component that handles collisions for GameObjects like dice and ice cubes. More info
  See in Glossary 2Ds and Polygon Collider 2Ds.

Use Auto Mass

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.