Epistemology (from the Greek episteme – knowledge) is a philosophical concept for the study of how we know the things we know. For skeptics it is an important concept that helps us ensure that we have a consistent process that is designed to ensure what we think we know conforms to reality. Philosophers break down knowledge to three key concepts, ‘Knowing That’ (4 is the answer to 2+2), ‘Knowing How’ (the process we use to arrive at 2+2=4) and ‘Knowing-Acquaintance’ (knowing a person or oneself). But what is the difference between knowledge, belief, faith, trust, doubt and how do they relate to truth? These are all part of epistemology. According to Plato, knowledge is a subset of that which is both true and believed.
Science and scepticism are closely linked. An objective scientist will always leave some room for doubt, despite her ‘knowing’ that evolution is a fact. So in what sense can she claim to know this if there is room for doubt? There is also the issue of the Gettier problem - Is Justified True Belief Knowledge? We can be right for the wrong reasons, reasons that we may not know are wrong. Gettier contended that while justified belief in a true proposition is necessary for that proposition to be known, it is not enough. It must be based in some external test of reality. Plato pointed out that we value knowledge over deduction. Knowledge is more valuable than mere true belief because it is tethered or justified. Justification, or working out the reason for a true belief, locks down true belief and makes it knowledge.
Our three paragraph ramblings are insufficient to dive into why we know what we know but if you want to explore further we recommend you start at the Wikipedia page and head from there.