SDL handles the actual graphics of the window/screen.  It draws the pixels, etc.  SDL works in a two-dimensional universe, because computer screens are two-dimensional.  You can do cool stuff in SDL, but it works best for two-dimensional games, like Pac-Man.

OpenGL handles the 3-d stuff.  OpenGL at its most basic is a Matrix library.  It handles basic translations, rotations, and projections of 3-d objects onto a two-dimensional plane. (the computer screen).  It handles the math for you.  So you don't have to (all that much).  It uses trignometry and matrix operations, built-in, you don't have to use them yourself all that much.

When you use OpenGL, you need to add a couple more libraries:  opengl32.lib and glu32.lib .  You probably already have them on your machine.

The 3-dimensional space used by OpenGl:  Usually the origin (0,0,0) is at the center (but you can change perspective).  Positive x is to the right; negative x is to the left.  Positive y is up.  Negative y is down.  Positive z axis is toward you.  Negative is z is away from you.  OpenGL graphics follow the right-hand rule.  Most 3D graphics libraries do (not all).