OnCollisionEnter is called when this collider/rigidbody has begun touching another rigidbody/collider.

In contrast to OnTriggerEnter, OnCollisionEnter is passed the Collision class and not a Collider. The Collision class contains information about contact points, impact velocity etc. If you don't use collisionInfo in the function, leave out the collisionInfo parameter as this avoids unneccessary calculations. Note that collision events are only sent if one of the colliders also has a non-kinematic rigidbody attached.

	function OnCollisionEnter(collision : Collision) {
		// Debug-draw all contact points and normals
		for (var contact : ContactPoint in collision.contacts) {
			Debug.DrawRay(contact.point, contact.normal, Color.white);
		}
		
		// Play a sound if the coliding objects had a big impact.		
		if (collision.relativeVelocity.magnitude > 2)
			audio.Play();
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnCollisionEnter(Collision collision) {
        foreach (ContactPoint contact in collision.contacts) {
            Debug.DrawRay(contact.point, contact.normal, Color.white);
        }
        if (collision.relativeVelocity.magnitude > 2)
            audio.Play();
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnCollisionEnter(collision as Collision) as void:
		for contact as ContactPoint in collision.contacts:
			Debug.DrawRay(contact.point, contact.normal, Color.white)
		if collision.relativeVelocity.magnitude > 2:
			audio.Play()

Another example:

	// A grenade
	// - instantiates a explosion prefab when hitting a surface
	// - then destroys itself
	var explosionPrefab : Transform;
	function OnCollisionEnter(collision : Collision) {
		// Rotate the object so that the y-axis faces along the normal of the surface
		var contact = collision.contacts[0];
		var rot = Quaternion.FromToRotation(Vector3.up, contact.normal);
		var pos = contact.point;
		Instantiate(explosionPrefab, pos, rot);
		// Destroy the projectile
		Destroy (gameObject);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Transform explosionPrefab;
    void OnCollisionEnter(Collision collision) {
        ContactPoint contact = collision.contacts[0];
        Quaternion rot = Quaternion.FromToRotation(Vector3.up, contact.normal);
        Vector3 pos = contact.point;
        Instantiate(explosionPrefab, pos, rot) as Transform;
        Destroy(gameObject);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public explosionPrefab as Transform

	def OnCollisionEnter(collision as Collision) as void:
		contact as ContactPoint = collision.contacts[0]
		rot as Quaternion = Quaternion.FromToRotation(Vector3.up, contact.normal)
		pos as Vector3 = contact.point
		(Instantiate(explosionPrefab, pos, rot) as Transform)
		Destroy(gameObject)