Angels are the divine mechanism by which God orchestrates all things in the multi-verse (Heaven, Earth and all the cosmos). Many Angels are also the "ministering spirits" for all divine affairs dealing with humankind. There are almost an infinite number of these celestial creatures. Holy Angels (good and benevolent beings) answer to and serve only God.
Likewise there are "fallen angels" who are the opposition to the Lord God (whom they still have to eventually answer to). They try and hinder the affairs of humankind in the world and stop us from reaching salvation with God.
I have just posted the first part of a series of HubPages that will cover this topic in much more depth... Enjoy and enlighten!