I'm having trouble understanding all the various theories of epistemology and how they relate, and which are considered current. Is there a good resource or reference to understand the current state of the field?
To understand current epistemology, you need to understand Gettier. Before Gettier, there was a fair amount of consensus on knowledge: it was justified true belief (JTB). Plato said it. There were specific problems over induction and rationalism vs empiricism and so on, but to understand the basic question of "what is knowledge?", that's easy—you believe something, that something is true, and your belief is justified (by some evidence or experience or whatever).
In the twentieth century, Edmund Gettier raised a problem with this: the Gettier problem. He showed a number of examples of situations where one satisfies the conditions given by the JTB account but cannot be said to have knowledge in any meaningful sense. (Russell also raised something like the Gettier problem in Problems of Philosophy, and suggested an interesting solution—see the Wikipedia page for details.)
Responses to the Gettier problem have drawn the lines of debate for current epistemology: between internalists and externalists, and between foundationalists and coherentists. All of these accounts try and add something to the justified true belief account. In the case of internalists, it is some kind of mental condition: to know that p you have to believe p; p has to be true; you have to be justified in believing p; and you have to be able to recognise by introspection of your conscious thoughts that you are justified. The internalists place the extra factor that makes justified true beliefs into knowledge within the mind. The externalists place it in the world.
There are a variety of forms of externalism: reliabilism is the most popular one. It says that you know something when you have a justified true belief arrived at through a reliable process. You will know that someone has scored a goal because the process of coming to learn that is a reliable one: if they had not scored the goal, you wouldn't end up having sensory perceptions of the goal being scored that induce justified beliefs. But if you are reading a newspaper that frequently prints propaganda or malicious lies, you would not have knowledge even if you have true beliefs because it is possible for the newspaper to not be telling the truth or only be telling the truth by accident. A perfect metaphor for reliabilism suggested by D.M. Armstrong in Belief, Truth, Knowledge is that of the thermometer. You know things when your beliefs change in response to the changing environment. The externalists run into interesting problems: without a mental property, one can end up saying that a computer or thermometer or database "knows" things.
For more information, check The Analysis of Knowledge on Stanford Encyclopedia of Philosophy.