According to mainstream christianity, an angel is a spirit creation that exists to serve God which is often portrayed in pop-culture looking something like this:
Could someone please give the Mormon definition of exactly what an angel is?
The Bible Dictionary distributed with LDS editions of the Bible contains a fairly complete doctrinal explanation of the LDS view of angels. A few key points:
Angels are of the same race of creation as humanity, distinct from mortal humans by being in a different phase of the eternal lifecycle:
These are messengers of the Lord and are spoken of in the epistle to the Hebrews as “ministering spirits” (Heb. 1:14). We learn from latter-day revelation that there are two classes of heavenly beings who minister for the Lord: those who are spirits and those who have bodies of flesh and bone. Spirits are those beings who either have not yet obtained a body of flesh and bone (unembodied) or who have once had a mortal body and have died and are awaiting the Resurrection (disembodied). Ordinarily the word angel means those ministering persons who have a body of flesh and bone, being either resurrected from the dead (reembodied), or else translated, as were Enoch, Elijah, etc. (D&C 129).
Not all angels are on God's side:
There are references to fallen angels in 2 Pet. 2:4 and Jude 1:6.
The scriptures speak of the devil’s angels. These are those spirits who followed Lucifer and were thrust out in the war in heaven and cast down to the earth. See Rev. 12:1–9; D&C 29:36–38; Moses 4:1–4; Abr. 3:27–28; and as alluded to by Peter and Jude cited above.
And contrary to the image posted above:
Angels do not have wings (HC 3:392).