Original: It is also used in video games, where it competes with Direct3D on Microsoft Windows platforms (see OpenGL vs. Direct3D).
Simplified: OpenGL is often compared to Direct3D, an API for 3D graphics on Windows. Programming for Direct3D is different in some ways because the APIs use different naming schemes for talking to a computer's graphics driver, which means that programmers need to write different code for OpenGL and Direct3D to do the same things.