MP is Multiprocessing - it is when you have multiple processors working in the same computer at the same time. There are a few forms of MP -
The different forms of multi-processing each have their advantages and disadvantages. Symmetrical generally means that every processor behaves like every other processor. Asymmetrical generally means that each processor is not the same as every other processor - so one processor may play traffic cop to the others, or different processors may each be handling special functions.
The first thing to realize about talking about Multi-Processing is that Hardware and Software people use the same terms to mean different things.
Advantages and disadvantages of each approach -
With AMP there is an advantage in that specialty tasks can finish especially fast - those processors (or tasks) are not loaded down doing basic system services or checking to see what else they should be doing. Those processors (or processes) are focused strictly on bashing out the one task they are supposed to do. But when they are done - they sit and wait for the main processor to give them something else to do, or they look from within their list of things to do next. Sorta like Union workers -- they are dependent on their supervisor.
SMP allows each processor to be more independent, but less specialized. If you load the System down, you load all the processors down. This generally means that each processor keeps itself more busy than in the AMP system.
For certain tasks the AMP system can just work better. For hardware - SMP only works well up to about 8 processors (realistically it is often less), after that they are fighting with each other for access to the shared memory so much that they get almost no real work done. (Kinda like two siblings doing the dishes -- "I wanna dry, you dry" - "No, you wash!"). Many of the largest MP systems are AMP, and for a reason. However, SMP systems do keep themselves more busy if you have less than a few processors - they are always looking for something to do, and trying to do it. Since they are not "specialized" they may take just a little bit longer than they would if they were specialists - because they also have to see if they should be doing something else. So they are busy as beavers, but not always quite as productive on the "one" task as if they were focused on that one task only.
Lets say I am doing a photoshop filter and running a word-processor (with spell checking), and doing a file copy -
For some jobs the current Mac behavior is exactly the behavior you want - especially time critical ones. This model is great for professional work like photoshop filters, renders, video filtering, sound, 3D, etc. However, your machine does not get the speed up as much (or at all) when not doing one of those tasks. The AMP (Mac) solution is a little more "all or nothing" - it is either a lot better at what you want to do, or it does not make much of a difference.
For now Apple has added support to the current MacOS so that many system services will automatically take advantage of multiple processors - tasks like QuickTime, QuickDraw or 3D, which are exactly the type of demanding tasks that most people want MP for. However there are many parts of the OS that it will not improve dramatically with the current Mac-MP - and many people focus only on that, when in reality the Power-Users (those most likely to need MP) are likely to be very satisfied with the results. In the future when MP becomes cheaper and more mainstream, then the MP support in Rhapsody will be SMP and will be better for those users as well.
Specialty (Dedicated Task) - MP
The Macs also have some specialty accelerators that are technically MP systems. Often designers put 4 (or more) computers on a specialty card, just for doing a particular task - often photoshop or multimedia or 3D work. These are sort of a special case. Often the processors on this card are SMP from inside the cards perspective - but the Mac is still the traffic cop. These systems do not run the MacOS and are completely dedicated to one or two tasks. These are usually high-end specialty systems, often using specialty processors (not PowerPC's) and are only used for professional work, and they are not even marketed as "MP" systems - just as "photoshop accelerator" cards or "trans-puters".
Multiple OS - MP
The Mac also has another kind of MP - multiple OS MP. Usually MP is done only running one Operating System and multiple tasks. But many Mac users put in a second processor that is an Intel Pentium and they run Windows or DOS or Unix on that second processor at the same time as they are running the MacOS. This allows them to run Windows apps very quickly, and copy the data back and forth. It also allows the users to start one task on one system - like say a file download off Internet, or do a compile or a search on one processor (that is behaving like a separate machine) - then go to their other machine and have a full speed computer available to them. (This design is usually an AMP system from both the hardware and software point of view - but would not be what people think of if you used that term).
Macs for now have a really interesting compromise between an SMP and AMP design that is pretty good for professional work doing specialty tasks. In the future Apple is going to a full SMP system, and any MP hardware bought now will work automatically when the OS is upgraded. The Macs also support another form of MP with mutli-OS's running simultaneous or dedicated task MP.
Windows 95 does not support any MP at all, and according to MS never will. Windows NT supports an SMP model - but usually only 2-processor models are sold (on the Mac there are many 4 processor variants). If you are using WinNT you lose many other advantages, like PnP and some Application compatibility - and there are many more issues with getting drivers for your hardware, etc.
The Macs also allow many users to upgrade to MP after the fact. Most Mac machines produced have a separate processor card (or two) that allow you to replace a single processor with 2 or 4 processors. With PC's you must replace your whole motherboard unless you bought an MP motherboard to begin with.
So on the Mac MP is real - MP is available now - and MP will get better in the future.