The drawing surface for OpenGL is generically referred to as the frame buffer. In actuality, the frame buffer might be a window created by your computer's window system or an in-memory data structure (like an X pixmap). OpenGL's frame buffer can logically be considered a set of buffers. A buffer is logically just a two-dimensional array of values. The most important buffer is the image buffer which contains the actual color information and possibly the alpha component but there are also other types of buffers. A window system might support multiple frame buffer configurations, each supporting different types of buffers. Multiple windows of different configurations can be displayed at one time though a single window has a fixed frame buffer configuration. In X, visuals are overloaded to also describe supported OpenGL frame buffer configurations.
The non-image buffers are often referred to as ancillary or helper buffers. While they do not contain the image itself, they can be essential in properly generating the image.