A core is basically a CPU, just physically a multicore cpu is in one package. Unless you are in the habit of touching them you can thing of a core as a cpu.
You may be told a lot of hog wash about cores, such as with multiple cores you can run multiple tasks. Well you can run multiple tasks on one core/cpu. Multiple cores mean that your operating system has to split up the work, this is not always possible. Therefore
2.6GHz + 2.6GHz < 5.2GHz, and
2.6GHz + 2.6GHz ≥ 2.6GHz. Yes multi core can sometimes be only as good as one core. It all depends on OS, and usage.
Readers note: The
+ sign above is not normal addition, it is core addition. That is why
2.6GHz + 2.6GHz = 2.6GHz is possible.
Edit: see Paul A. Clayton's comment, it points out that multicore can at times be better than single, but only when all cores can be utilised (multi-thread/multi-process).