C# isn't an API, it's a language. OpenGL is an API because it's a specification for a set of functions that you can call to achieve a goal (graphics being drawn on the screen in the case of OpenGL), but there isn't actually any OpenGL code. For that, you need a library that implements the OpenGL API, such as mesa3D. A library is the file that contains the code that runs when you call the functions in an API. The I in API is for interface, so think of C# interfaces vs. classes (an interface in C# is roughly equivalent to a class with only purely virtual methods in C++).
oh i get it, its a high level language not an API, im using a visual for the first time, so it must have libraries in it that make it seem like an API too, its pretty simple, i can make stuff my mum could use :D
I dont know whats wrong with me but im still having trouble understanding :/ visual c# is a language because when i type something i got a whole list of pre-made commands made out of other commands that do stuff, and they are all stored somewhere and i get to choose one on a little menu system thingy.
sdl is an API because the keywords that do stuff do a whole lot more than smaller code and they allready been made up