DirectX is a set of APIs (Application Programming Interface) that programmers use to create multimedia software (mostly games) on Windows platforms. It encapsulates access to 2D and 3D by software or hardware accelerated means. It also provide access to sound, joystic and network in a unified form following the COM standard. It is composed of:
DirectDraw[]
Its function is to display 2D graphics. It is basically a blitter plus some advanced features like hardware acceleration and scaling. A blitter (bit block transfer) is a part of the videocard that makes bit block tranfers from the memory of the card to the screen, displaying graphics. When a blit is made something is displayed on the screen, examples of this are the main character of the game, the enemies and the background. Check blitter article to see some programming examples.
DirectDraw[]
Its function is to allow 3D hardware acceleration or software emulation, in case the card does not have it. All new videocards support 3D rendering and this API.
DirectSound[]
Its function is to allow access to accelerated or non-accelerated sound. It allows reproduction of simultaneous sounds, frequency control, etc.
DirectInput[]
Its function is to provide an interface for input peripherals like keyboard, mouse, joystick and steering wheels, among others.
DirectPlay[]
Its function is to allow an interface for multiplayer games. On these games, several players play on different computers connected to a network, which can be a WAN or a LAN.