Epistemology essentially deals with the question of how 'knowledge is acquired', and how reliable that knowledge is. In essence it is the theory of knowledge, and has focused on how knowledge relates to concepts such as truth, belief, and evidence. This is a very general topic, and is constantly debated.
A good place to start in the study of Epistemology is the definition of knowledge. A simple and old answer is "a justified, true belief." However, Edmund Gettier famously argued against that definition.