It's the end of the world as we know it, and I feel confused.

Instead of a lone doomsday-sayer on a city street corner with a sandwich board, the news media and popular culture have taken to talking about the end being nigh. Especially with the 2012 Mayan calendar predictions, the big one has become big business. But not all end-of-days scenarios are equal within the world of dedicated apocalypse nerds.

There are multiple theories that go beyond the zombocalypse or rise of the machines, and even the words themselves that are used to describe humanity's last hurrah have different meanings, depending on the groups that use them.

To understand the nuances of the language of the end times, we compiled the following key phrases from the nerd set as a glossary to go out on, along with the help of John R. Hall, a professor of sociology at the University of California, Davis, and author of "Apocalypse: From Antiquity to the Empire of Modernity."

Al-Qaida

An apocalyptic-minded group (or "apocalyptic warring sect," according to Hall) within Islam seeking to bring on the end of this world and the beginning of the next. Al-Qaida believes that "only through the exercise of violence can God's plan for a new age be realized," and its members see themselves as God's agents, Hall said. The same definition may apply to actors within the Puritan Revolution in the 17th Century (Oliver Cromwell) and the Crusades, he said.

Apocalypse

In Greek, "apocalypse" means something like "disclosure of things previously hidden." "That being the title of the (Christian) Book of Revelation, it's often taken to mean the revelation of the last things before God's final judgment," Hall said. Apocalyptic movements emerge many times through history where profound changes are said to occur and a new world is dawning. But, Hall added, "those movements are not generally about the end of the world in the ultimate sense."

Armageddon

"End times" is simply a popular usage for talking about the end of the world, and the same thing can be said for "doomsday." "Armageddon" is a specific reference to the Book of Revelation or apocalypse in the Bible's New Testament. Said Hall: "It is the final battle between good and evil that comes just before God's judgment at the end of the world."

Big crunch

This is essentially the opposite of the big bang. This theory among cosmologists posits that the constant expansion of the universe will one day stop and reverse, and the universe will collapse until it becomes a black hole.

Books of Chilam Balam

These Mayan books from the 16th to the 18th centuries speak about history, myths, medicinal recipes, daily life, war and politics -- along with some apocalyptic predictions. In 1951, astronomer and linguist Maud Makemson, a recipient of the Guggenheim fellowship for the study of Mayan astronomy, released "The Book of the Jaguar Priest."

The book contained her translation of "The Book of Chilam Balam of Tizimin" from the 1500s, which presented a Mayan end-of-world scenario on Dec. 21, 2012: "Then the god will come to visit his little ones. Perhaps after death will be the subject of his discourse." Makemson also translates that, "in the final days of misfortune, in the final days of tying up the bundle of the 13 [cycles] ... then the end of the world shall come" and "these valleys of the earth shall come to an end." Then again, the Maya may have believed that the end of one world was followed by the beginning of a new one.

Camping rapture

Evangelist and religious broadcaster Harold Camping predicted the Rapture would take place on May 21, 2011, which would have righteous believers of Jesus Christ rising up to heaven, followed by a five-month period of torment on Earth brought on by the Four Horsemen of the Apocalypse. On Oct. 21, 2011, the world was then supposed to end. Camping had previously made predictions for 1988 and 1994. "Harold Camping is probably the most successful date-setting apocalypticist the world has yet seen," Hall said.

Catabolic collapse

A theory by John Michael Greer, an author and "Grand Archdruid of the Ancient Order of Druids in America," catabolic collapse refers to what happens when civilization can't meet the demands for the stuff it produces. When maintenance needs cannot be met by available resources, societies may cut back for a bit and are impoverished, then return to business as usual, or they could begin to amass even more stuff for the next round.

Greer suggests that we're at the end of a big collapse, largely because of fossil fuel consumption. He is, however, optimistic that as this current cycle of civilization comes to a close, it will re-emerge as a radically different, newly optimistic one.

Eschatology

A belief or theology regarding the end of humanity or the end of the world.

Galactic superwave

The superwave theory by physicist Paul LaViolette, Ph.D., in the book "Earth Under Fire" posits that as spiral galaxies detonate on a periodic basis, the explosions can expand to encompass our entire outer space neighborhood. Not only would we not know of an impending superwave for thousands of years after it began, but the explosion would probably bring a lot of debris that would cover up our sun.

LaViolette also thinks a superwave might be heralded by a gravitational wave and then a gamma-ray burst. He suggests that the Asian tsunami on Dec. 26, 2004, was actually caused by a gravitational wave and said the largest gamma-ray burst ever recorded occurred on Dec. 27.