Angels are a creation of God which were created a little higher than man.  We find this in the book of Psalms in a description of our Lord Jesus Christ when He was a man.

What is man, that thou art mindful of him? and the son of man, that thou visitest him?  For thou hast made him a little lower than the angels, and hast crowned him with glory and honour.  [Psalms 8:4-5]

The word Angel means messenger.  The Bible does not describe the exact nature of Angels but rather describes their relation to God, Jesus Christ, and man.  Angels are given a work to do which is to be ministering spirits.  This is described in the book of Hebrews.

And of the angels he saith, Who maketh his angels spirits, and his ministers a flame of fire.  [Hebrews 1:7]

Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?  [Hebrews 1:14]

The exact function of God's messengers are not spelled out in specific detail.  Rather they are presented as doing Gods bidding, whatever that may be.  They appear as helpers and protectors of people in need, as proclaimers of news, delivering revelations for God, and as guides and guardians. 

Some people believe Angels are people that have died and gone on to heaven.  While this cannot be found in the Bible, there is a promise that we will be made equal to the angels after the resurrection.  This is found in Luke when the Sadducees questioned Jesus on how marriage would work after people are resurrected. 

And Jesus answering said unto them, The children of this world marry, and are given in marriage:  But they which shall be accounted worthy to obtain that world, and the resurrection from the dead, neither marry, nor are given in marriage:  Neither can they die any more: for they are equal unto the angels; and are the children of God, being the children of the resurrection.  [Luke 20:34-36]

There are two classes of Angels.  There are the good Angels who follow God's will.  There is also a second class of fallen Angels which have sinned and been cast out of Heaven along with their leader the devil.  They are deceitful, familiar spirits whose work is to hinder salvation.  This can be found in the book of Revelation.

And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, And prevailed not; neither was their place found any more in heaven.  And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.  [Revelation 12:7-9]

These fallen angels are hard at work today trying to turn mankind from the teachings of Christ.

Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;  [1 Timothy 4:1]

Angels are creations of God placed in the divine order between man and Christ and are numerous.  There are good angels who follow the will of God and bad angels who follow Satan.  Angels are marvelous creatures, encompassed with splendor and glory.  They are a God given source of inspiration of strength, inspiration, and encouragement to the faithful.