The name Abaddon or Apollyon appears in Revelation 9:11: "They had as king over them the angel of the Abyss, whose name in Hebrew is Abaddon, and in Greek, Apollyon." In Hebrew, the name "Abaddon" means "place of destruction"; the Greek title "Apollyon" literally means "The Destroyer."
In Revelation 8-9, John describes a period during the end times when angels sound seven trumpets. Each trumpet signals the coming of a new judgment on the people of earth. When the fifth angel blows his trumpet, the Abyss, a great smoking pit, will open, and a horde of demonic "locusts" will rise out of it (Revelation 9:1-3). These creatures will be given the power to torture any person who does not bear God's seal (verse 4). The pain they inflict will be so intense that sufferers will wish to die (verse 6). Abaddon/Apollyon is the ruler of the Abyss and the king of these demonic locusts.
Abaddon/Apollyon is often used as another name for Satan. However, Scripture seems to distinguish the two. We find Satan later on in Revelation, when he is imprisoned for 1,000 years (Revelation 20:1-3). He is then released to wreak havoc on the earth (verses 1-8) and ultimately receives his final, eternal punishment (verse 10). Abaddon/Apollyon is likely one of Satan's underlings, a destroying demon and one of the "rulers," "authorities," and "powers" mentioned in Ephesians 6:12.
John Bunyan's classic allegory The Pilgrim's Progress includes a memorable scene in which Christian does battle with a demonic monster named Apollyon. True to its name, Apollyon nearly destroys Christian. The pilgrim in his armor withstands the attack and wields his sword to repel the fiend. Bunyan's "Apollyon" is a symbolic representation of our spiritual enemy, but the inspiration for the character is literal. The Abaddon/Apollyon of Revelation is a real being who will one day inflict real pain on real people during God's real judgment.