From the 
Skeptics Society's FAQ:
What is a skeptic?
What does it mean to be a skeptic? Some people believe that skepticism is rejection of new ideas, or worse, they confuse “skeptic” with “cynic” and think that skeptics are a bunch of grumpy curmudgeons unwilling to accept any claim that challenges the status quo. This is wrong. Skepticism is a provisional approach to claims. It is the application of reason to any and all ideas—no sacred cows allowed. In other words, skepticism is a method, not a position. Ideally, skeptics do not go into an investigation closed to the possibility that a phenomenon might be real or that a claim might be true. When we say we are “skeptical,” we mean that we must see compelling evidence before we believe. Skeptics are from Missouri—the “show me” state. When we hear a fantastic claim we say, “that’s nice, prove it.”
Skepticism has a long historical tradition dating back to ancient Greece when Socrates observed: “All I know is that I know nothing.” But this pure position is sterile and unproductive and held by virtually no one. If you are skeptical about everything, you would have to be skeptical of your own skepticism. Like the decaying subatomic particle, pure skepticism uncoils and spins off the viewing screen of our intellectual cloud chamber.
Modern skepticism is embodied in the scientific method, that involves gathering data to formulate and test naturalistic explanations for natural phenomena. A claim becomes factual when it is confirmed to such an extent it would be reasonable to offer temporary agreement. But all facts in science are provisional and subject to challenge, and therefore skepticism is a method leading to provisional conclusions. Some claims, such as water dowsing, ESP, and creationism, have been tested (and failed the tests) often enough that we can provisionally conclude that they are not valid. Other claims, such as hypnosis, the origins of language, and black holes, have been tested but results are inconclusive so we must continue formulating and testing hypotheses and theories until we can reach a provisional conclusion.
The key to skepticism is to continuously and vigorously apply the methods of science to navigate the treacherous straits between “know nothing” skepticism and “anything goes” credulity. Over three centuries ago the French philosopher and skeptic, René Descartes, after one of the most thorough skeptical purges in intellectual history, concluded that he knew one thing for certain: Cogito ergo sum—I think therefore I am. But evolution may have designed us in the other direction. Humans evolved to be pattern-seeking, cause-inferring animals, shaped by nature to find meaningful relationships in the world. Those who were best at doing this left behind the most offspring. We are their descendents. In other words, to be human is to think. To paraphrase Descartes:
Sum Ergo Cogito—I Am Therefore I Think.
See also: The Skeptical Manifesto.
One common misunderstanding is the assumption that skeptics are only skeptical about positive claims, that is the more skeptical you are the less you believe, and if you crank your skepticism to the max you end up believing nothing. That is simply not the case.
When a skeptic evaluates a claim, they also automatically evaluate the logical inverse of the claim. For example, when examining the claim "extraterrestrial life exists", they also simultaneously look at "we are alone in the universe". When evaluating the probability that "artificial intelligence is possible" they also evaluate the probability that "artificial intelligence is impossible", knowing that the respective probabilities have to add up to one. This examination of opposite claims creates a dynamic tension, with evidence and implications pushing on both sides. So the more skeptical you are the more likely you will arrive at an accurate understanding of the reality of the claim.