Answer:
Epistemology (the philosophy of knowledge) breaks down into two categories:
A true belief.
A justified true belief.
The first has been referred to in many different ways over the millennia: dogma or axiom being ones that most people recognize.
The second form of knowledge exposes the basic problem; the infinite regress. A chain of justifications must either be infinite (and, it cannot be infinite, for obvious reasons), or have a starting point that does not require justification, which gets us the first form.
Different philosophers have attempted to solve the infinite regression problem in order to do away with the first form of knowledge (mostly because they just have a visceral dislike of it: it smacks of religion).