Angels are Spirits created by God. Just as
He created everything else in this universe
He created the beings we call Angels. 
Their numbers cannot be totaled.
It says in scripture "Those who are with
us are more than those who are with
them." 2Kings 6:16 

Angels are the permanent residents of the
heavenly realms. They come to our realms
for a specific purpose, but they return to 
heaven when their work is done.

Angels do not have a body as we perceive
of them, but they can take any shape or
form they need to in order to do their work. 

The Nature of an Angel is Spirit. But a
Spirit may not be an angel. A Spirit
becomes an Angel when he receives a
commission from God, and when that
commission is finished, he becomes a
Spirit again. 

In other words, Angel is the office or the
job the Spirit does, not the Spirit itself. 

Angels can sin, (eg the fallen angels), but
the Glorious Angels of God who reside in
Heaven have not sinned. God is not called
Father by the holy angels because there
has been no provision for their salvation.
The Fallen Angels fell before there were
sinners and there was no provision for
them, therefore their sin cannot be forgiven. 

Angels are not Heirs of God, only
Christians are heirs with Jesus Christ
through their acceptance of His redemptive 
work on Calvary. (Romans 8:17) The Angels will stand aside when the Church of Christ comes to take it's rightful place at Christ's side in eternity. 

Angels have no personal testamony of 
Salvation and they have no experience of
the indwelling Holy Spirit, they will never
experience it. They also do not need the
ministry of the Holy Spirit as people do.
They are endowed with authority by a
continuing relationship with God through
their obedience and continued sinlessness. 
We need to be given authority other than 
our own which was broken by sin. Therefore 
we receive the Holy Spirit when we accept
Jesus as our Personal Savior. 

Angels have far greater power then men,
but they are not omnipotent or omniscient. 
But they are refered to as the "mighty angels 
of God" by Paul in 2Thessalonians 1:7,
and Peter states that "angels who are a 
greater in might and power do not bring 
a reviling judgement". (1 Peter 2:11) 
It took only one angel to destroy all the 
first born of Egypt (Exodus 12); 
only one angel to close the lion's mouth
and save Daniel (Daniel 6:22);
only one angel to save Shadrach,
Meshach and Abednego from the flames
of the furnace (Daniel 3:28). 
But even at that, their powers are limited by God.