OpenGL (Open Graphics Library) is a cross-language, multi-platform application programming interface (API) for rendering 2D and 3D vector graphics.OpenGL is managed by the non-profit technology consortium Khronos Group.Opengl is same as Directx graphics language from microsoft.
OpenGL is used on various OS such as windows,linux,mac os etc. Use of OpenGL graphics lanugage is free. Opengl graphics language is used for various graphics design in game development , CAD , Information visulization,flight simulation and much more. You can generate 2D or 3D graphics with OpenGL. OpenGL is same as C programming language and it is free of cost. It is easy to use for everyone. OpenGL was developed by Silicon Graphics Inc.(SGI) from 1991 and released in january 1992 according to WIKIPEDIA. recently latest version of OpenGL 4.4 relesed at 22 july,2013. It has many new feature. We can easily learn OpenGL if we know basic about C programming language. OpenGL is fully hardware base programming language and best preffer to NVIDIA graphics card. There are many tutorials available on the websites and also youtube videos. C and C++ programming language does not provide graphics. If you want to use graphics in your application which is written in C/C++ programming language then you can use Opengl graphics library. You can also use Opengl graphics with JAVA and other supported programming language.
If you want to learn OpenGL graphics programming than here is a link for tutorial and referance OpenGl book.
Here is a link for a red book of OpenGl
www.opengl.org
Here is a link for tutorials of OpenGl
www.videotutorialsrock.com
www.glprogramming.com