WHAT is the difference between a sceptic and a denier? When I call myself a sceptic, I mean that I take a scientific approach to the evaluation of claims. A climate sceptic, for example, examines specific claims one by one, carefully considers the evidence for each, and is willing to follow the facts wherever they lead.
A climate denier has a position staked out in advance, and sorts through the data employing "confirmation bias" - the tendency to look for and find confirmatory evidence for pre-existing beliefs and ignore or dismiss the rest.
Scepticism is integral to the scientific process, because most claims turn out to be false. Weeding out the few kernels of wheat from the large pile of chaff requires extensive observation, careful experimentation and cautious inference. Science is scepticism and good scientists are sceptical.
Denial is different. It is the automatic gainsaying of a claim regardless of the evidence for it - sometimes even in the teeth of evidence. Denialism is typically driven by ideology or religious belief, where the commitment to the belief takes precedence over the evidence. Belief comes first, reasons for belief follow, and those reasons are winnowed to ensure that the belief survives intact.