Someone who believes that knowledge is not possible for themselves, some to different extents. They may believe that knowledge is possible, but paradoxically, it's impossible for them to know. While one will confine this to the metaphysical, most famously, regarding the existence of God, others will believe that they can know nothing at all, as not even their senses can be trusted.
Agnostics are often well-educated, and even more often do they see themselves as such. An agnostic, by virtue of their belief, typically sees themself as a very skeptical and critical person, and indeed, they usually are. They are often more intelligent on average than those who believe they know things, or at least, so the stereotype goes. Because of this, they often develop a sense of superiority, by which they can logically and rationally transcend the arguments of less-intelligent theists, who tend to be the most vocal ones.
An agnostic esteems above all else the scientific method. It is the infallible absolute of the agnostic. Everything must be empirically demonstrable to an agnostic, or it cannot be known to exist. A typical argument an atheist would use against a theist would be: "Since you cannot prove the existence of your God, how do you know that he exists?"
An agnostic attempts to apply empiricism
to absolutely every aspect of reality. A thing does not exist unless it can be "scientifically proven". A modern understanding of science is the typical rhetoric of the agnostic.