What makes someone an antipope? Rather straight forward question. There have been multiple men declared antipopes. Is it different each time, or generally the same set of circumstances that causes this?
Frank K. Flinn's Encyclopedia of Catholicism defines an antipope as "a person who lays claim to the office of bishop of Rome and tries to act as head of the Roman Catholic Church, in opposition to the person duly holding that ecclesiastical office in the eyes of the church as a whole."  Flinn points out that there have been 39 antipopes in the history of the Catholic Church and that the criteria for papal validity as well as the correct electoral body for the popes has evolved over time.

The first person declared an antipope was St. Hippolytus of Rome (r. 217-35) for his challenging of Pope Callistus (r. 217-222).  The best known antipopes are those from the Babylonian Captivity when the seat of the papacy was moved in 1309 from Rome to Avignon, France where it remained until 1377--these popes were known as antipopes by the Holy See in Rome and a valid pope was elected by that body.  

For further reading, you can refer to:
Richard P. McBrien, Lives of the Popes (San Francisco: HarperSan Francisco, 2000)
Daniel MacCarron, The Great Schism: Antipopes Who Split The Church (Dublin: D.M.C. Universal, 1982).

I hope that helps answer your question some!

All information in this reply came from:
Encyclopedia of Catholicism. Frank K. Flinn. New York: Facts on File, 2007. 33-34.

Full Disclosure:  I am an MSIS student at the University of Texas' School of Information and am answering this for a class on reference.  I found this source through the Gale Virtual Reference Library.