AGP improves the process of storing texture maps by allowing the operating system to designate RAM for use by the graphics card on the fly. This type of memory is called AGP memory or non-local video memory. Using the much more abundant and faster RAM used by the operating system to store texture maps reduces the number of maps that have to be stored on the graphics card's memory. In addition, the size of the texture map your computer is capable of processing is no longer limited to the amount of RAM on the graphics card.
The other way AGP saves RAM is by only storing texture maps once. It does this with a little trickery. This trickery takes the form of a chipset called the Graphics Address Remapping Table (GART). GART takes the portion of the system memory that the AGP borrows to store texture maps for the graphics card and re-addresses it. The new address provided by GART makes the CPU think that the texture map is being stored in the card's framebuffer. GART may be putting bits and pieces of the map all over the system RAM; but when the CPU needs it, as far as it's concerned the texture map is right where it should be.