How Transistors Work - A Quick and Basic Explanation

Do you know how transistors work? What is it about transistors that make them the building block of so many electronic items, like computer, laptops, smartphones and tablets these days?

What is a transistor? The transistor is arguably the greatest invention of the last century, or perhaps, one of the greatest of all time. If it wasn't for transistors, we couldn't have developed smaller, faster and cheaper computers, telephones, GPS devices, radios, gaming consoles, smart televisions, routers and a device -- no larger than a human hand -- that integrates all these marvels under the same screen.

This is a short, layman explanation of what is a transistor and how it works.