An angel is a spiritual being created by God to be His servant, and also to serve those who will inherit salvation, as Scripture says. However, Satan and 1/3 of the angels rebelled against God and were cast down from heaven, with Satan causing mankind to sin. Though Jesus in Scripture implied that the angels in heaven could not have sex or father children, it is also suggested through what Jesus had said to Satan in "Glorious Appearing" that the angels that fell with Satan fathered children through the "daughters of men" in the time of Noah that produced the Nephilim.
There were angels that appeared throughout the Left Behind books that provided deliverance and protection for the believers in Christ during the Tribulation period and also during the Millennial Kingdom. The Angel making the most appearances in the Left Behind series is named Anis, who appeared during Buck Williams' attempt to smuggle Tsion Ben-Judah out of the country of Israel to the safe house next to the New Hope Village Church, when Buck and Chaim Rosenzweig visited the empty tomb of Jesus Christ after Leonardo Fortunato had incinerated Hattie Durham, and when Rayford Steele was kidnapped by members of The Only Light.
Archangel Michael appears in a dream to Tsion Ben-Judah after the midpoint of the Tribulation, and then again in minor appearances during the second half of the Tribulation; his final appearance is during Kingdom Come, in which he wrestles Satan to a pin, and then imprisons him in the lake of fire.
Other Angels shown in the series include Nahum, Christopher, and Caleb, who make an appearance ministering to a group of Muslim rebels in The Remnant. Archangel Gabriel shows up briefly in Glorious Appearing in his canonical function as herald.
Angels are shown to wield great powers, but have few if any desires or ambitions on their own -- in Kingdom Come: The Final Victory, they are described as "not qualified as guests" during the Feast of the Bridegroom at the beginning of the Millennial Kingdom.
Powers shown include selective invisibility, enough telekinesis to remotely open cell doors, and the ability to utter a soporific blessing that can cause unprepared humans to fall asleep instantly. They can also cause human metabolism to temporarily freeze, resulting in momentary paralysis and gap in short-term memory. Finally, they have been shown to generate the illusion of becoming much larger than their physical body is.
Rather than being impervious or resistant to projectile weapons, they are shown to be unaffected by them; bullets and even missiles pass through them without an appreciable loss of momentum. Whether this is due to a phasing ability or due to the fact that the Angel witnessed by Mac McCullum during Operation Eagle was a telepathic or holographic projection is unknown, although since his and Rayford's stories agreed and they appeared on camera, the latter is more likely. It is also unknown whether Angels are impervious or resistant to Directed-Energy Weapons.
In the Bible, they are also shown capable of making limbs go numb with a touch. Genesis 32:22 shows that a (possibly the only) critical weakness of an Angel is the inability to match a trained human fighter in stamina in a fair fight.