The critical difference is that skeptics are open to evidence, whereas denialists have made up their minds are just looking to justify their beliefs. By my understanding, for example, apologists are denialists.

The skeptic will want to know more about a particular thing, before coming to a conclusion whether firm or tentative. He may or may not acquire that requisite knowledge. The denialist, OTOH, before he has anything approaching a sufficient understanding is either dismissing evidence or exaggerating it in accordance with his preconceived emotional attachments.