Scripting Reference
Random.Range

static function Range (min : float, max : float) : float

Description

Returns a random float number between and min [inclusive] and max [inclusive] (Read Only).

JavaScript 
// Instantiates prefab somewhere between -10.0 and 10.0 on the x-z plane 
var prefab : GameObject;
function Start () {
    var position: Vector3 = Vector3(Random.Range(-10.0, 10.0), 0, Random.Range(-10.0, 10.0));
    Instantiate(prefab, position, Quaternion.identity);
}
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public GameObject prefab;
    void Start() {
        Vector3 position = new Vector3(Random.Range(-10.0F, 10.0F), 0, Random.Range(-10.0F, 10.0F));
        Instantiate(prefab, position, Quaternion.identity) as GameObject;
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public prefab as GameObject

    def Start():
        position as Vector3 = Vector3(Random.Range(-10.0F, 10.0F), 0, Random.Range(-10.0F, 10.0F))
        (Instantiate(prefab, position, Quaternion.identity) as GameObject)

static function Range (min : int, max : int) : int

Description

Returns a random integer number between min [inclusive] and max [exclusive] (Read Only).

If max equals min, min will be returned. The returned value will never be max unless min equals max.

JavaScript 
// Loads a random level from the level list

Application.LoadLevel(Random.Range(0, Application.levelCount));
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        Application.LoadLevel(Random.Range(0, Application.levelCount));
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        Application.LoadLevel(Random.Range(0, Application.levelCount))