That being said, Angels are in the Bible and there is an entire field of biblical study on the topic of angels called angelology. (Much of angelology is spent knocking down misconceptions held by society.) While most of what the Bible says about angels could be handled in a single post, this post will only deal with a couple questions.
What, or who are angels?
Angels are beings created by God. Often they are unseen, but when seen they look like lightning or fire, or they seem to have the ability to look like humans (2 Kings 6:15-17, Genesis 18:2-19:22; John 20:10; and Acts 12:7-10 for example). Hebrews 13:2 even suggests that they can blend in and be completely mistaken for humans. In these cases, it seems that angels don't have wings; however, we must also remember verses like Isaiah 6:2 where an angelic being called a seraphim is said to have six wings. In other accounts we see an angelic being called a cherubim. This is the being that's waiving a flaming sword back and forth to prohibit man's reentry to the Garden of Eden and the Tree of Life (Genesis 3:24). The cherubim is also the same creature God commanded the Hebrews to sculpt on top of the Ark of the Covenant. These cherubim had wings that touched each other (Exodus 25:17-22). Demons are fallen angels, cast out of heaven and waiting for the final judgment and not granted forgiveness or salvation through repentance (see 2 Peter 2:4; Jude 6).
There is nothing in the Bible that suggests that angels were ever human. We do not become angels when we die and our deceased loved ones are not angels looking over us. In addition, angels do not become humans; they are not our future family members in some kind of preexistence waiting for a body on earth. The Bible does not speak of angels or humans in this way and there's nothing suggesting that humans were in a preexistence with God. These ideas are simply creations of human thinking. The Bible teaches that humans are the pinnacle of God's creation, not angels (to see this, start reading in Genesis 1 and stop after Revelation 22).
What do angels do; this is, what is their purpose?
Just as is the purpose of man, angels were created to glorify God. We often see angels worshiping God (Psalm 103:20-21, Psalm 148:2, and Isaiah 6:1-7 for example). Sometimes they act as God's messengers such as in Daniel 8-9 and Luke 1. They protect God's people (Psalm 34:7; Psalm 91:11, and Acts 12 for example). Matthew 18:10 seems to suggest that children have an angel watching over them and Luke 16:22 might suggest that angels have a responsibility at the time of a believer's death. And most importantly, angels usher in and proclaim Christ at his birth, resurrection, and return. Angels don't die and they they do not marry (Matthew 22:30; Luke 20:35-36).
Too often, people get hung up on the work of angels. In doing so, they completely miss the bigger work of God as he is redeeming his creation. Looking to angels, they do not look upon Christ. In order to see angels rightly, it is best to first see Christ for who he is. (If you have questions, I am happy to answer them and chat more about this with you. You may contact me here.)
* Photo of mourning angel at the churchyard of San Miniato al Monte (Firenze) in Firenze, Italy was taken by Mark Voorendt, April 2001 and is registered under a creative commons license.