OpenGL is as the name implies an open source graphics API (application programming interface). It provides hardware rather than software rendering meaning it accesses the graphics card and provides quality rendering not bound by the CPU. It is almost always used for scaling vector graphics both 2D and 3D and is available on every common operating system today.
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OpenGL is not commonly used for high-octane graphics like you see in triple-A gaming titles (recent example: Overwatch). Rather it's used wholly or in part is independent game development in graphics-heavy normal applications or in simpler games. For any game that doesn't demand high polygon counts though as well as for tasks like emulation it works very well and provides extremely high speeds. It's not hard to learn given how common it is and software using it can generally be ported from one operating system to another making it extremely flexible. Odds are you've used OpenGL software more than once often without knowing it. Its longevity having been first released 24 years ago speaks to its quality and it's still maintained today. If you need to install an OpenGL fixture to make an application work do so knowing you're in good hands. If you want to program something with 3D graphics and don't know where to start this particular API is a great choice and you'll find use for it again and again.