OpenGL - What is this exactly?

I've never gotten around this thing because I don't really understand this tech stuff. Googling gives me results and explanations in such a way as if they assume I already know what it is.

Can soembody explain in simple language how exactly is this different when playing games? Like I got a choice of taking DirectX or OpenGL. What's the difference? Also somebody told me that OpenGL doesn't work on hardware but rather on software - or something like that, can't remember properly.

In a nutshell, OpenGL is an opensource version of DirectX.

Most apps only support or run better on DirectX than on OpenGL. Only in rare cases OpenGL is better than DirectX.

OpenGL does run off your hardware.