Full Idea
The classical Five Modes of Scepticism are Discrepancy (people always disagree), Relativity ('according to you'), Infinity (infinite regress of questions), Assumption (ending in dogma) and Circularity (end up where you started).
Gist of Idea
Scepticism can involve discrepancy, relativity, infinity, assumption and circularity
Source
Michael Williams (Problems of Knowledge [2001], Ch. 5)
Book Reference
Williams,Michael: 'Problems of Knowledge' [OUP 2001], p.61
A Reaction
I take Relativity to be different from scepticism (because, roughly, it says there is nothing to know), and the others go with Agrippa's Trilemma of justification, which may have solutions.