Besides being the platform that makes services like Netflix and Facebook possible, the cloud is – in simple terms – little more than a big group of computers.
The “little more” part that makes those computers into a “cloud” is specialized management software.
This software allows ten, a thousand or hundreds of thousands of computers to function more or less as if they were just one (much bigger) computer. If you want a computer with the power of 10 computers, the cloud can do that. If you want a computer with the power of 150,000 computers, the cloud can do that too.