Skepticism is a word used to refer to a variety of positions. There are three main types of skepticism: philosophical skepticism, or Pyrrhonism, scientific skepticism, and religious skepticism. Sometimes these different varieties of skepticism may overlap. In common use, the word "skepticism" most often refers to scientific skepticism.

Scientific skeptics question certain claims by testing them through experiment. Common claims targeted by scientific skeptics includes the efficacy of homeopathy and other New Age therapies, the existence of ESP/telekensis and telepathy, the plausibility of supernatural beings (ghosts, poltergeists, angels, and gods), cryptozoology such as the Loch Ness Monster and Bigfoot, alien visitations and associated phenomena, creationism, conspiracy theories, claims of pop psychology, and more. The task of the scientific skeptic is to debunk "cherished nonsense" — claims that continue to be believed despite ample evidence to the contrary.

Philosophical skepticism has a more formal and abstract meaning than scientific skepticism. Philosophical skeptics refrain from making any statements about the absolute truth of things, believing that anyone can be wrong. The originator of philosophical skepticism is the ancient Greek philosopher Pyrrho of Elis (c. 360-275 B.C.), and it is sometimes called Pyrrhonism in his name. Philosophical skepticism can even be turned on itself, meaning philosophical skeptics may reserve judgment on whether the entire platform of philosophical skepticism is valid or not.

A religious skeptic is a person who doubts certain religious claims, such as miracles, the appearance of angels, the omnipresence of God, etc. A religious skeptic need not be an atheist or agnostic; he or she may simply be a believer doubtful of certain religious claims. For instance, many Christians are skeptical that the Virgin Mary would really reveal herself in a tortilla, and blame such sightings on overactive pattern recognition.

Within scientific skepticism, there are activist skeptics who consider it their duty to debunk fraudulent claims. Some of the most famous activist skeptics include comedians Penn and Teller, the magician James Randi, and the hosts of the TV series Mythbusters.