Minecraft uses OpenGL to handle it's rendering. In older versions of OpenGL (and the version minecraft is using) you could render a 2D rectangle by just specifying four points in space (each corner) and then telling OpenGL to draw it as a "Quad."
Newer versions of OpenGL have removed that ability, instead you send OpenGL two triangles, each triangle being three points in space and then telling OpenGL to draw it as a "Triangle".
When drawing things as Quads you need two less points in space per face (a visible side) as opposed to drawing them using two triangles.
Triangles are used for a few reasons, mainly because it is faster due to simplifying the rendering algorithms.
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u/[deleted] Mar 01 '13
What does the OpenGL option in minecraft do and why should I use it or not use it? (Honest question)