Put very simply, a skeptic is someone who is committed to logic, reason and critical thinking when making assessments or decisions.
Skeptics understand all knowledge to be tentative and data dependent. No matter what we may believe to be true, if the data contradict that belief, it is wrong. The world is as it is, not as we might wish it to be.
Along with empiricism and rationality, skepticism forms a critical part of the bedrock of science. The methods of science help us steer a course between an extreme "we can know nothing" skepticism and total credulity in which virtually anything may be accepted as valid or true.
Skeptics tend to be an optimistic bunch who have a positive outlook on life. While the cynic expects negativity and disaster at every turn, the skeptic approaches life with a questioning attitude and a desire for objective evidence in support of claims made about the world. The more extravagant the claims, the greater the demand for clear supportive data.
Skeptics are sometimes accused of closed-mindedness, a criticism which is strongly rejected. The skeptic is open to the possibility of refutation of favoured ideas.
The position most commonly taken is that current scientific knowledge is the best we have and that science, while limited, is the best tool we have for learning about the world. At the same time, we must be open to change in the future as science progresses.
Without this position of openness, we cannot hope to move forward and we risk rejecting ideas which are potentially accurate and useful. If however, we are too accepting and not questioning enough we may end up spending a lot of time and energy on nonsense. The key is to find a good balance between these two extremes.
No. People are sometimes concerned that skeptics are, by definition, anti-religious. This is not the case.
There are many skeptics who have religious beliefs and who distinguish clearly between questions concerning the material world (the realm of science), and a non-material world in which they believe and for which there is no physical evidence. Such belief is a matter of choice or faith and the majority of skeptics accept and respect this.
There are however, areas of overlap. If claims are made in the name of religion that concern physical phenomena and that can be tested, skeptics may engage in such testing. For example, claims of statues moving or of miraculous appearances or cures may be challenged.