terminology - What is the meaning of dual core vs. dual cpus?

What is the difference between multiple CPU processors (ie: Pentium D) and multicore processors (ie: Core 2 duo)?

Answer

Multiple CPU processors (like the Pentium D) are literally just two distinct CPUs (two dies) that just happen to share the same physical package.

So, instead of having 2 CPUs in 2 sockets, you have 2 CPUs that just happen to fit in one socket together. Basically, there's no difference between having, say, 2 physical early-generation Xenon CPUs (each in their own socket) and a single Pentium D. (Note: This is very much a simplification.)

Multi-core CPU processors (like the the Core 2 series) are two processing units that share the same die (silicon substrate).

So, the multiple CPU processors are like having 2 CPUs which are just conveniently packaged together, whereas the multi-core CPU processors are 2 CPUs on the same chip.

If this seems like a subtle distinction, that's because it is.

Of course, there's more to it than that - for one thing, having the two CPUs on the same die is more efficient (both power-consumption-wise as well as instruction-wise, due to a different internal design). Additionally, having the CPU cores on the same die means they can share things like L1 cache, whereas when the two CPUs are physically separate dies, they each have to have their own cache.

It gets even more complicated when you throw quad-core chips into the mix, because they are 2 dual-core chips which just happen to share the same physical packaging... kind of like the Pentium D. But that's a digression for another day.