Lwjgl OpenGL Help

Question:
Question;
I've made my top-down minecraftlike game to a point. Dynamic/adjustable types, basic entities, player movement, a worldgen...
However.
This whole time I didn't understand that I needed to code the game to use a Video card via OpenGL and stuff, so the game lags like a whore by only using CPU power to display everything.
So, I'm looking into lwjgl(Yes, I'm using Java), and following some tutorials I understand 'kinda' how to draw basic 2D shapes using OpenGL.
However, I am stuck.
There seem to be multiple ways of displaying a graphic?
DisplayLists, and Vertex Buffers?
I have NO idea what I am doing here.
Do I somehow have the video card calculate the layers of images I am making?
Or is it 'purely' for display?
Sorry if the answer seems obvious, but although I know coding enough to do alot of stuff, this OpenGL stuff seems way over my head and I'm not exactly sure what computer logic I am trying to follow.

>>5
Can somewhat large, well put together game actually run okay without using OpenGL?
I talked to someone about the lag before, I gave them a rough idea of what I was doing with everything, and they just chalked it down to needing to do some work with OpenGL and let the graphics card do some of the work.
I guess I can set up a profiler. I don't have one yet, but I can imagine it would work something like comparing times between some things and see how much lag happens between them all.
Thanks for your replies so far.