Angels are created beings. There are warring angels, ministering angels and angels sent specifically to protect us. There are many different kinds of angels, but they really have only one purpose, and that is to help us.
Some people think an angel's job is to defend God, but God does not need defending. If God were to raise His little finger, then boom!—whatever He wanted would be done. God just thinks a thought, and it's done. He doesn't need defending.
So angels are here to help us. They are the links between heaven and earth. God's Spirit reveals Himself to us, and His angels are His logistic agents. So they help us get from one place to another. They protect us from certain things and events. They protect us from things that the enemy, Satan, wants to happen. Our goal is to live a life that allows God to be justified in sending angels to help us.
Some angels are sent to help other angels. For example, angels could be sent to help the angel who has been assigned to you. Some people have more than one angel because of the callings God has on their lives and because of the difficulty of what they are going through or will be going through.