An interface developed by Intel which allows a graphics card to access memory directly which along with the wider bandwidth helps improve graphics performance.
AGP has now come to replace a lot of PCI video cards due to its superior performance, most modern motherboards have an AGP slot as standard. Some motherboards come with an AGP graphics card built-in.
You will see AGP cards advertised with different specifications, including throughput (bandwidth), here is a summary of the throughput they refer to:
1X - 266MBps
2X - 533MBps
4X - 1.07GBps
8X - 2.1GBps
Obviously the higher the throughput the better, although nowadays most video cards run at 8X.