What is a guardian angel? I have always heard of guardian angels and have probably even prayed to them, but never really knew anything about them.  What is the purpose of guardian angels and where does the belief in them come from?
The concept of guardian angels  and praying to them to protect one is deep rooted in the Catholic Church. 
Many other churches and Christians also believe in existence of guardian angels based on the following Bible references:

Psalms 91:11 (NLT)


  For he will order his angels
        to protect you wherever you go.


This gives the idea that God will ask His angels to protect each person who trusts in Him on his/her way.

In Matthew 18:10 (NLT) Jesus says


  Beware that you don’t look down on any of these little ones. For I
  tell you that in heaven their angels are always in the presence of my
  heavenly Father.


This verse is often taken to mean that there is an assigned angel for each of the follower of God and those angels are in the presence of God. Many take both references cited above to mean that a single angel is assigned for a single believer. I believe rather it implies there is an assigned angel for each believer and that angel may be assigned for one or more living persons who trusts in the God.

More indirect references can be found else where in Bible. Like for instance, Hebrews 1:14 (NLT) implies that one role of angels is to care for people.


  Therefore, angels are only servants—spirits sent to care for people
  who will inherit salvation.