How is Mesa different from OpenGL drivers?

I believe LinuxQuestions.org forum member geeman2.0 provided a good explanation for this matter:

OpenGL and Mesa aren't really two separate choices, but rather Mesa is a specific type of OpenGL.

OpenGL is simply an interface that defines a standard set of functions needed for drawing 3d graphics. It doesn't involve the actual code that makes these functions happen, it only specifies what the functions are called and what they are supposed to do.

An openGL implementation provides the actual code that runs the methods specified by the OpenGL standard. Without an implementation installed, you cannot run any opengl programs.

Mesa is just one of many OpenGL implementations, and is the most standard one included in linux distributions. It does all the work in software, which is why it is slow.

When you install Nvidia or ATI drivers for a fancy graphics card, these drivers provide a new OpenGL implementation that runs on the graphics card. This implementation would take the place of the Mesa implementation, but it is still an openGL implementation. That is, all of Mesa/ATI/NVidia drivers implement the same set of functions, they just do it in different ways, and they are all openGL.

Source

Can someone clarify the distinction between Mesa and drivers?

Mesa provides the client side OpenGL interface for the open source GPU drivers based on the DRI2/DRM architecture. Or in other words: It's also a part of a driver.

If you've got the proprietary drivers from NVidia or AMD installed you don't need Mesa. If you want to use the open source drivers (nouveau, radeon, radeonhd, intel) you need Mesa.