Clueless about stocks

Ok I know this may sound a bit stupid but...
What is a 'stock'? Is it like a paper or certificate? And what is it like in the investment world? How do you invest?

'A stock is a certificate that shows that you own a small fraction of a corporation. When you buy a stock, you are paying for a small percentage of everything that that company owns, buildings, chairs, computers, etc. When you own a stock, you are referred to as a shareholder or a stockholder. In essence, a stock is a representation of the amount of a company that you own.'

Source: http://library.think...