[TLAM Strike]

AGP is the slot the card fits in. PCI is the other common one.

Look at this image:
http://upload.wikimedia.org/wikipedi...x-AGP_slot.jpg
the two on the bottom are PCI the one on the top is AGP (althogh most AGP slots are brown.)

AGP vs PCI
As computers became increasingly graphically-oriented, the graphics card became far more important than other PCI devices, and, thus, the AGP slot was developed. AGP slots are superior to PCI for graphics cards because they provide a dedicated pathway between the slot and the processor, allowing for faster communication between the two. In addition, AGP uses sideband addressing, meaning that addressing for packets is carried outside of the packet, meaning that the entire packet does not need to be read to get addressing information. In addition, to load a texture, a PCI graphics card must copy it from the system's RAM into the card's framebuffer. An AGP card is capable of reading textures directly from system RAM using the Graphics Address Remapping Table (GART). GART reapportions main memory for texture storage, allowing the graphics card to access them directly.

The two main reasons graphics cards with the PCI interface are produced is that firstly they can be used in nearly any PC, as very few modern desktop PCs do not have PCI slots - though some motherboards with built-in graphics adaptors lack an AGP slot. Secondly, a user with an appropriate operating system can use several PCI graphics cards (or several PCI graphics cards in combination with one AGP card) simultaneously - to give many different video outputs (for the use of many screens). This cannot be done with AGP cards, as very few (if any) motherboards are equipped with more than one AGP slot.