AMD uses Hypertransport to connect to the Northbridge (northbridge connects to the rest of the system, ie HDDs and GFX), and has a direct link from CPU to Memory.
Intel uses a FSB (front side bus) to connect to the Northbridge, which then connects to Memory (and of course it connects to all the rest of the system too).
AMD's advantage is when you have a bunch of HDDs or GPUs and whatnot; because AMD has two busses it doesn't get bogged down so often because the memory can't take any bandwidth from the rest of the system, and vice versa. Also (and this is the main thing people talk about) having a direct CPU to memory link lowers latency because it's a shorter path from CPU to memory than CPU to Northbridge to memory.
Look up Hypertransport and Front Side Bus on Wikipedia, they have pictures there