@[email protected] 's answer is a very good and complete one !

On a side note, I would add that this is a problem that kinda divides philosophers (and people more generally). From my own experience, it is very common to discredit skepticism as a useless philosophy/reflexion, maybe because it seems to be pointless to a lot of people. But it has been used through the whole history of philosophy :

• In Ancient Greece, there were different Schools of Skepticism, as explained in other answers
• It was quite common during Renaissance, typically with Montaigne, or Descartes (though he was using a skeptic method to assert the existence of knowledge)
• To me, skepticism is one of the influences that led to the emergence of philosophies like phenomenology, as they try to describe our experiences as subjects without asserting facts about an external world.
• As i studied logic and epistemology, i discovered that skepticism has influences in those fields. For exemple, the epoche, the suspension of judgment used by skeptics, is used in modern epistemology, with theories such as Equal Weight View. They also talk a lot about limitations of language, formal logic being some slight kind of remedy to it.

So in the end, while i feel like skepticism has a really bad reputation, i also find very common to accept the idea that every truth/belief/knowledge/reflexion needs unprovable premises, which seems to me like a skeptic root. It is not often the main subject, but i frequently find signs of it around me.