What is the difference between a CPU and a vCPU?

Some VPSs quote a number of CPUs and others quote vCPUs?

How does the vCPU differ from the CPU? Do they refer to what a VPS is guaranteed,or what a VPS is guaranteed if they are available?

Do they have their CPU equivalents, eg Amazon mention in the past that a CPU was the equivalent of 1.7GHz Opteron or some other AMD CPU.

http://ift.tt/1dvHQ0Z