What was Skepticism?
Skepticism was a Hellenistic philosophy. Like other Hellenistic philosophies, it was interested in understanding how people could live tranquil lives. Skeptics (like Sextus Empiricus) from this period argued that the best way to live in tranquility was to understand that we simply cannot know anything.
The Skeptics believed that objects in the world have no real truth to them. Things have no stable characteristics. Because they have no stable characteristics, we cannot possibly know anything about them. Because we cannot know things, we should simply stop trying to understand. Once we do this, we will be tranquil.
This is, of course, a difficult philosophy to assert because it claims to know that things are unknowable. For this reason, the Skeptics did more in the way of attacking other philosophers than of trying to put forth their own ideas.