What is Abaddon and Apollyon?

The fifth angel sounded his trumpet, and I saw a star that had fallen from the sky to the earth. The star was given the key to the shaft of the Abyss. When he opened the Abyss, smoke rose from it like the smoke from a gigantic furnace. The sun and sky were darkened by the smoke from the Abyss. And out of the smoke locusts came down upon the earth and were given power like that of scorpions of the earth. They were told not to harm the grass of the earth or any plant or tree, but only those people who did not have the seal of God on their foreheads. They were not given power to kill them, but only to torture them for five months. And the agony they suffered was like that of the sting of a scorpion when it strikes a man. During those days men will seek death, but will not find it; they will long to die, but death will elude them.  Revelation 9:1-6

The falling star is an angel (meteors don't open shafts with keys) which opens up the Abyss.  It unleashes a swarm of locusts, it appears.  They don't harm Christians, just non Christians.

The locusts looked like horses prepared for battle. On their heads they wore something like crowns of gold, and their faces resembled human faces. Their hair was like women's hair, and their teeth were like lions' teeth. They had breastplates like breastplates of iron, and the sound of their wings was like the thundering of many horses and chariots rushing into battle. They had tails and stings like scorpions, and in their tails they had power to torment people for five months.  Revelation 9:7-10

These locusts looked like horses, with gold crowns, human faces, long woman’s hair, lion’s teeth, iron chests, wings, scorpion tails that sting.  If you take it literally, it becomes mythology.

Are they demons?  Mostly likely it's an army.  Locusts appear in the Old Testament:
  • Exodus – one of the plagues
  • Isaiah, Jeremiah – armies likened to
  • Joel – 835 BC, speaks of a locust plague that devastated the area and compares it to a greater judgment coming soon, an army.  Known also as The Day of the Lord.
Joel uses language very similar to Revelation.

 1 Blow the trumpet in Zion;
       sound the alarm on my holy hill.
       Let all who live in the land tremble,
       for the day of the LORD is coming.
       It is close at hand-

 2 a day of darkness and gloom,
       a day of clouds and blackness.
       Like dawn spreading across the mountains
       a large and mighty army comes,
       such as never was of old
       nor ever will be in ages to come.

 4 They have the appearance of horses;
       they gallop along like cavalry.

 5 With a noise like that of chariots
       they leap over the mountaintops,
       like a crackling fire consuming stubble,
       like a mighty army drawn up for battle.

 7 They charge like warriors;
       they scale walls like soldiers.
       They all march in line,
       not swerving from their course.

 11 The LORD thunders
       at the head of his army;
       his forces are beyond number,
       and mighty are those who obey his command.
       The day of the LORD is great;
       it is dreadful.
       Who can endure it?

 28 "And afterward,
       I will pour out my Spirit on all people.
       Your sons and daughters will prophesy,
       your old men will dream dreams,
       your young men will see visions.

 29 Even on my servants, both men and women,
       I will pour out my Spirit in those days.

 30 I will show wonders in the heavens
       and on the earth,
       blood and fire and billows of smoke.

 31 The sun will be turned to darkness
       and the moon to blood

       before the coming of the great and dreadful day of the LORD.  Joel 2:1-5,7,11,28-31

If John is footnoting Joel, then the closest association to these locusts points to an army.  Joel warns, not of locusts, but of the destruction of Judah.  It’s all going to be wiped out soon.  Wouldn’t John be talking about an army too wiping out Jerusalem?  They key is found in the meaning of these two words.

They had as king over them the angel of the Abyss, whose name in Hebrew is Abaddon, and in Greek, Apollyon.  Revelation 9:11

Abaddon means "destruction"
1) ruin
2) destruction
3) the place of destruction
4) the name of the angel-prince of the infernal regions, the minister of death and the author of havoc on the earth

Apollyon means "Destroyer"
1) the angel of the bottomless pit, the Destroyer

This is the only place these words appear.  Together they mean a destructive force from hell.  That destructive force from hell (Abaddon and Apollyon) will swarm Jerusalem and destroy it.