Both DirectX and OpenGL are collections of application programming interfaces (APIs) used for handling multimedia information in a computer. They are the two most common APIs used today for video cards.
DirectX is created by Microsoft to allow multimedia related hardware, such as GPU, to communicate with the Windows system. OpenGL is the open source version that can be used on many operating system including Mac OS.
The decision to use DirectX or OpenGL APIs to program is dependent on operating system of the target machine.
Unreal Engine 4 was first launched using DirectX11. Following the announcement that DirectX 12 ships with Windows 10, Unreal has created a DirectX 12 branch from the 4.4 version to allow developers to start creating games using this new DirectX 12.
An easy way to identify APIs that are a part of DirectX is that the names all begin with Direct. For computer games, the APIs that we are most concerned about are Direct3D, which is the graphical...