Then he will send out the angels, and gather his elect from the four winds, from the ends of the Earth to the ends of Heaven.

According to the bible, angels are messengers of god. They praise his name, they carry out his wish, they "see" after the humans. When we luckily avoided something bad, we speak of "having a guardian angel". But when one want's to know more about angels, it really starts getting complicated. I will try to give a basic introduction to that field of interest.
Angelos in Greek means ambassador, which denominates a profession, as Jesus himself was called "messenger of the covenant" (Mal 3.1). The primordial Christian belief in angels as "beings" rather than personified aspects of god relates back to the beginning of Christianity.
The creation of angels happened BEFORE the 6-days (Hiob 38,4-7, 2.Petr. 2,4; i.Mos. 2,15). So, angels are not the souls of the dead !
Now, angels are very very old, they are not eternal, but were created (exept for Jesus, who was the "angelus increatus", the "angel without creation", thus god himself)
Today, most of us know the archangels Gabriel, Michael, Raphael. And of course Luzifer, the fallen angel. But there are 7 archangels in Judaic and christian belief, 4 in Muslim belief.
Angels are hierarchically ordered in 9 Choirs or Orders as follows: