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I'm new to the game, so this may be a dumb question, but I can't figure out if there's any difference between a Warlock and an Epic Warlock in terms of how you're allowed to use them?

In a nutshell, the metastory of Warmachine/hordes is one that moves forward and changes. In some cases, some warcasters/warlocks go through some sort of an event in the story that makes them have a epic form. There is no real difference on how they when it comes to building an army on paper (like you are not forced to take X amount of points to use this caster or only a certain type of unit, warjack, or solos in order to use the Epic Warcaster), but an epic form of a Warcaster/warlock does sometimes have some different ways how it plays so you have a different play-style with the same character, so you want to build your army based around his/hers strengths and weaknesses.

A little bit shorter answer is no not really, except the play-style in comparison to the prime and epic warcaster. Possibly might be over-explaining it so someone might explain it better than me.

No difference, they each use the same rules for army building.

Not in Mk2. They're just different versions of the same named character with different rules(representing story progression). You can use any of them whenever.

In Mk1 you needed to be at a certain point level to use Epic level warcaster. Now its only a mechanism to represent character development down the storyline.

The only real rule interaction is that, in character restricted formats, you can only have 1 of a specific character in both your lists combined. But that is a rare format.