The word ‘scepticism’ can refer to any of a set of philosophical approaches which are, to varying degrees, pessimistic about our ability to attain knowledge. Sceptics may deny that our current beliefs about a certain subject are justified, or they may make the even stronger claim that any we may have beliefs about a certain subject cannot in principle ever be justified. Different varieties of scepticism are distinguished by the degree of pessimism they exhibit towards knowledge, and the range of beliefs that this pessimism is directed towards.
Skepticism: useful introductory piece from philosophy.about.com
Skepticism Introduction: a TheoryOfKnowledge.info series on skepticism
Skepticism: a great introductory article from Philosophy Basics
Skepticism: Stanford Encyclopedia of Philosophy article covering key philosophical disputes related to skepticism
History of skepticism: a useful article discussing the historical development of skeptical thought