A skeptic is "someone who habitually doubts accepted beliefs". There are many kinds of skeptics in society. Some are harmless, some are funny, some are purely annoying and a few are dangerous.
The Casual Skeptic (or ignorant skeptic) is someone who doesn't know anything about a field but has preconceived opinions on it and, refusing consciously or unconsciously, to get to know the subject, simply doubts and dismisses it. These are harmless skeptics and we probably all have been casual skeptics on some subjects. Casual Skepticism can be useful when some incredible and dangerous claim is made and you don’t care about it. For example when people claimed that at 11:59:59PM on December 31st, 1999 the Earth would come to an end, most of us where “Casual Skeptics”. Most of us (including me) said “whatever”… and went back to the usual business. Without a little bit of Casual Skepticism we would be worrying about everything and anything and our life would be sucked away from us.
The Scientific Skeptic is someone who has notions on a subject, researches the subject extensively and based on that knowledge doubts it but is also willing to change mind if he discovers new evidence or proof. The Scientific Skeptic search actively for such evidence or proof and does not wait for somebody to "bring it" to his attention. This kind of skepticism is healthy and is at the base of the scientific method (make a hypothesis and research and be open for the proof).
The Professional Skeptic is someone who made a career out of laud disbelief of some specific field or claim. These individuals life is dependent on their abilities to find holes in someone's belief or claim, feed other disbelievers with assumptions and rhetoric reasoning and they will not change their mind no matter what. Their career depends on been skeptic! A change of their mind requires a change in career. These kinds of skeptics are usually either annoying or funny and often are also Absolute Skeptics (see below). Their followers are mostly Casual Skeptics that stumble randomly across their articles, and after briefly agreeing with them move on forgetting all about it. Some of their followers are also the Absolute Skeptics, who love to add some more arguments under their belt to prove their point during their campaigns. Professional Skeptics will claim scientific evidence, but the evidence they bring on the table is carefully filtered to prove their points. They will simply ignore any other evidence (even if such evidence seems convincing even to them).
The Absolute Skeptic (or super skeptic) is the only really harmful type. This kind of skeptic, which is most often also a professional skeptic, claims to know everything in the field, but in reality knows only targeted information that confirms his disbelief. The Absolute Skeptic will study the evidence that confirms the claim and will try to find holes in it, even to the point of making up facts or fabricate assumptions. He/she knows everything that dismisses a claim and absolutely refuses to acknowledge the validity of anything that proofs the claim. If something seems to be proving the claim beyond any doubt, the absolute skeptic will attack the source of such information, will make up evidence against it or will refuse to acknowledge that such proof even exists. The absolute skeptic will attack any believer of the claim trying to destroy his or her image and reputation in any possible way. Some examples can be easily found in history. When Galileo Galilei claimed that the Earth revolves around the sun, the Catholic Church attacked him to the point of forcing him to claim that he was wrong. The Church acted as an Absolute Skeptic in that context. We see examples of absolute skepticism in politics, in science and in many other fields. These people decide (consciously) to take a position and do not and will not hear or admit (consciously) that anything else can be even remotely considered right. Absolute skeptics are a plague of humanity and their blindness often slows down human progress and scientific discoveries. Be aware of Absolute Skeptics!!