A computer processor, or microprocessor, reads and executes program instructions. The instructions are bits of data that tell the computer what to do.
The processor speed, also called the "clock rate," is measured in megahertz, MHz, or gigahertz, GHz. One megahertz (MHz) is one million hertz, and one gigahertz (GHz) is one billion hertz.
While it's true that a faster clock rate generally means the computer will run faster, there are other factors that impact the overall performance of the computer like how much memory the computer has and how many sets of instructions it is trying to execute simultaneously.
The core is the part of the processor that performs the instructions. The first computers had one core, meaning they could only process one instruction at a time, though computer makers found ways to speed them up so they were multitasking. A dual-core processor has two processing units that work together to process instructions. A multi-core processor has two or more cores and can process multiple instructions at one time. The more cores a processor has, the faster it is, though, software and other factors can affect performance.