A core is technically a single CPU.
A CPU, or “Central Processing Unit”, is the circuitry that actually performs calculations and executes the instructions of computer programs. You may think of it as the brain or engine of your computer.
Most desktop computers originally used a single processor chip containing a single core or CPU. A computer may now also have a single processor chip with multiple cores, multiple processors with single cores, or multiple processors with multiple cores. Regardless of the configuration, multiple cores allow a computer to quite literally do two or more things at the same time.
The term “CPU” is often also used to refer to the entire processor chip and all the cores it may contain.