How do i make my arrow hit an enemy and instantiate a blood sprite in its place?

I have researched about this by myself but I have always gotten confused between which to use,ontriggerenter or oncollisonenter?And how to use them?Can someone please shed light on this?

Here is my arrow code:

 > using UnityEngine; using
 > System.Collections;
 > 
 > public class PlayerArrowUp :
 > MonoBehaviour {
 > 
 >     //speed of bullet
 >     float speed;
 > 
 >     // Use this for initialization
 >     void Start()
 >     {
 > 
 > 
 >         speed = 2f;
 >     }
 > 
 >    
 > 
 >     // Update is called once per frame
 >     void Update()
 >     {
 > 
 >         //Get the bullets current positon
 >         Vector2 position = transform.position;
 > 
 >         //compute the bullets new position
 >         position = new Vector2(position.x, position.y + speed
 > * Time.deltaTime);
 > 
 >         // Update the bullets position
 >         transform.position = position;
 > 
 >         Vector2 max = Camera.main.ViewportToWorldPoint(new
 > Vector2(1, 1));
 > 
 >         //If the bullet went outside the screen,the destroy the bullet
 >         if (transform.position.y > max.y)
 > 
 >             Destroy(gameObject);
 > 
 >     } }

And here is my enemy code:

 > using UnityEngine; using
 > System.Collections;
 > 
 > public class
 > Monster_Movement_Downwards :
 > MonoBehaviour {
 >     public GameObject Monster_Blood;
 >     public int life = 0;
 >     //speed of bullet
 >     float speed;
 > 
 >     // Use this for initialization
 >     void Start()
 >     {
 > 
 > 
 >         speed = 1f;
 >     }
 > 
 > 
 > 
 > 
 > 
 > 
 > 
 >     // Update is called once per frame
 >     void Update()
 >     {
 > 
 > 
 > 
 > 
 > 
 > 
 > 
 >         //Get the monster's current positon
 >         Vector2 position = transform.position;
 > 
 >         //compute the monster's new position
 >         position = new Vector2(position.x, position.y + speed
 > * -Time.deltaTime);
 > 
 >         // Update the monster's position
 >         transform.position = position;
 > 
 >         Vector2 max = Camera.main.ViewportToWorldPoint(new
 > Vector2(1, 1));
 > 
 >         //If the monster went outside the screen,the destroy the monster
 >         if (transform.position.y > max.y)
 > 
 >             Destroy(gameObject);
 > 
 >     }
 >    
 > 
 > }