DirectX is a multipurpose library.
Probably what you meant was Direct3D.

Direct3D and OpenGL are API's that allow you to render information (pixels or vertex data) to the screen. Depending on your hardware, they may or may not, use hardware accelleration (the video card.)

The objective of these APIs (Direct3D or OpenGL) is to render dynamic (i.e. program-generated or changing) images to the screen. The images might come from a file, the user, or be randomly generated through perlin noise, but the main point is that the program gets to say what gets drawn where and when, allowing (ever-changing) positions stored in memory to be represented onscreen as hundreds of (moving) sprites in your favorite RTS game.