Direct
Graphics Access (or
Architecture, depending on whom you ask). Basically, a method for an
X11 program to access the
frame buffer (for novices: the
screen) directly. On the plus side you get faster graphics - and the
full screen mode too. On the negative side, you need to be
r00t to do this.
I don't know of the actual implementation details - to users, DGA often means "that hard and somewhat crashy way of getting programs to run on full screen, well, maybe not as fatally crashy as SVGALIB". I have heard (from Graphics Muse interview of Dirk Hondel) that in DGA mode in XFree86 4.0, the frame buffer itself is essentially just a pixmap that the program can mess with, using normal X11 pixmap library routines.
DGA is a part of X11's X Acceleration Architecture (XAA). See also DRI.