Sceptics (or Skeptics) fall into two categories.
The first category is called "philosophical scepticism" and it is a belief that knowledge is not truly possible, hence they effectively "doubt everything". This is not a particularly popular belief system any more, but it was quite popular a few hundred years ago.
The other, and most common, form of scepticism is called "scientific" or "rational" scepticism. These are people who have the attitude that all claims of "truth" or "fact" must be supported by impartial and verifiable evidence. Rational sceptics refuse to accept knowledge purely based on opinion or unsupported claims.
Sceptics are well known for dismissing what is termed as "pseudoscience", which are fields of study which do not have any evidence to support their validity (eg. homeopathy, astrology). Many sceptics are also dismissive of religious beliefs, due to the inherent difficulty (or impossibility) in proving the existence of God, miracles, etc.