Plato (circa 427-347 B.C.E.) was the first Western philosopher to consider in some detail the nature of knowledge and the way it is obtained.
As is frequently the case with philosophical theories, Plato's is centered on a metaphor. In his dialogue Meno, Plato points out that there is no practical difference between knowledge and correct opinion. If your opinion is correct, it will serve as a useful guide to action as well as knowledge. The difference between them is that opinions are transitory while knowledge is secure.
True opinions are a fine thing and do all sorts of good so long as they stay in their place, but they will not stay long. They run away from a man's mind; so they are not worth much until you tether them by working out a reason. . . . Once they are tied down, they become knowledge, and are stable. That is why knowledge is something more valuable than right opinion. What distinguishes the one from the other is the tether.
In the Theaetetus, Plato proposed that knowledge is true belief with an account. This is what has become known as the "traditional" analysis of knowledge. Unfortunately, the ensuing discussion of the nature of an account is of little relevance to contemporary theory of knowledge. Also of little contemporary interest is Plato's description in the Meno and Phaedo of how we obtain knowledge through "recollection" of what was known before we were born.
A listing of Plato resources found on the Web can be found at Episteme Links