HyperTransport is a high speed point-to-point interconnect for high performance components on a motherboard. It was designed primarily by AMD, and has been released in their flagship Opteron and Athlon64 desktop processors.
The HyperTransport architecture has allowed AMD to move away from the traditional SMP technology for mutiple CPU systems. In a HyperTransport system, each CPU has a number of HyperTransport links it can use to communicate to other CPUs, memory banks, and other high-performance components. This mitigates the bus overhead present in "standard" SMP systems, and with AMD's current technology allows up to 4 CPUs to have dedicated links to each other and to their own bank of memory.
See also: the HyperTransport homepage