The CPU is the "Central Processing Unit" of your VPS, which performs all necessary calculations. A CPU core is a processing unit which reads in instructions to perform specific actions.
The number of CPU Cores specifies the number of server processors that are available to your VPS. This is the main factor influencing the speed at which your VPS can perform actions. The underlying physical CPU´s of your VPS are Intel Xeon processors. The exact type and clock rate of these processors can vary. Within your OS you will be able to see a generic CPU + speed, but the virtualisation software of your VPS will make sure that every virtual CPU core can perform approximately the same amount of instructions per second.