SMT AND SMP

SMP (Symmetric Multiprocessing)

In theory SMP means that all CPUs are identical. In practice there may be minor variations between CPUs (e.g. different revisions of the same family of CPUs), but these differences can usually be ignored by operating system software. SMP is the easiest form of multiprocessing for software to support. SMP includes systems with CPUs implemented in separate chips, systems with CPUs implemented in the same chip (multi-core) and combinations (e.g. a system with 2 separate quad core chips, with a total of 8 physical CPUs).

SMT (Simultaneous Multithreading)

Normally a CPU executes one stream of instructions, where at any time some parts of the CPU may be busy and other parts of the CPU may be idle (e.g. while doing integer instructions parts of the CPU that do floating point calculations may be unused), and where the entire CPU may be idle in some cases (e.g. when the CPU needs to wait until data arrives from RAM before it can continue). The idea behind SMT is to execute more than one stream of instructions, which increases overall performance by reducing the chance that parts of the CPU become idle.
From the operating system's perspective, SMT looks a lot like SMP, except for performance characteristics. For SMT a single physical CPU executes multiple "threads" (or logical CPUs), and the performance of one thread/logical CPU is effected by work done by other threads/logical CPUs in the same physical CPU/core (as the physical CPU's resources are shared).
For the 80x86 architecture, Intel is the only manufacturer that has implemented SMT (called "hyperthreading" by Intel) so far. When Intel first introduced hyperthreading it got some negative publicity due to performance problems; partly caused by the way it was implemented and partly because a lot of software wasn't designed for it (single-threaded) and operating system schedulers weren't optimized for it. Intel stopped using SMT/hyper-threading for a while; but since then software has caught up and later CPUs (Atom, Core i7) show significant performance improvements from SMT (up to about 30% performance improvement on multi-threaded loads).

AMP (Asymmetric Multiprocessing)

AMP means that at least one of the CPUs are different. For example, an embedded system might have an ARM CPU and a special purpose CPU for digital signal processing. AMP is rare for desktop/server computers, however this may change in future as Intel has plans to put stream processors ("Larrabee", mainly intended as a GPGPU to be used for graphics, physics and HPC) in the same system as general purpose 80x86 CPUs (possibly even in the same multi-core chip). Possibly, at some time in the future, it may be necessary for a general purpose 80x86 operating system to have 2 schedulers - one for traditional 80x86 CPUs and another for stream processors.