Drawing surfaces receive video data to eventually be displayed on the screen as images (bitmaps, to be exact). In most Windows programs, you get access to the drawing surface using a Win32 function such as GetDC, which stands for get the device context (DC). After you have the device context, you can start painting the screen. However, Win32 graphics functions are provided by an entirely different part of the system, the graphics device interface (GDI). The GDI is a system component that provides an abstraction layer that enables standard Windows applications to draw to the screen.
The drawback of GDI is that it wasn't designed for high-performance multimedia software, it was made to be used by business applications like word processors and spreadsheet applications. GDI provides access to a video buffer in system memory, not video memory, and doesn't take advantage of special features that some video cards provide. In short, GDI is great for most business applications, but its performance is too slow for multimedia or game software.
On the other hand, DirectDraw can give you drawing surfaces that represent actual video memory. This means that when you use DirectDraw, you can write directly to the memory on the video card, making your graphics routines extremely fast. These surfaces are represented as contiguous blocks of memory, making it easy to perform addressing within them.
For more detailed information, see Surfaces.