Doubting everything and doubting one thing are different projects wearing the same name.
In ordinary conversation, calling someone “skeptical” just means they want more evidence before agreeing. Philosophy uses the same word for something much stronger: an argument that a whole category of belief, possibly even all belief, cannot meet the standard required to count as knowledge at all.
Descartes’ method in the Meditations doubted everything he could possibly doubt, his senses, the external world, even mathematics, to find one thing immune to doubt, the cogito. That targeted project is now called Cartesian skepticism when the doubt is aimed at a specific class of claims, such as the external world, while leaving a subject’s own mental states knowable. Pyrrhonian skepticism, the older tradition, goes further and denies that even that fallback is safe, treating all belief as unjustified and recommending suspension of judgment instead.
A structural problem shows up in Descartes’ own attempt to answer his own doubt: he uses reason to argue his way out of doubting reason, a circularity philosophers still call the Cartesian circle. Arguments like this matter beyond the classroom; they are the standard test any theory of knowledge has to survive, including ones that ground knowledge in revelation rather than pure reason alone.
Source: Stanford Encyclopedia of Philosophy, “Skepticism” (plato.stanford.edu/entries/skepticism).