Stocks!

Let's start off with corporate structure vocabulary, because I just so happen to have a sheet of it!

Formal Definition: A share of ownership in a corporation.

My Definition (slightly longer..(but more fun!)): Think of a company as a pizza (yum!). The job of this company/pizza is to make money by improving itself, and usually to do that it requires some money. So in order to get that money, the president of the pizza might sell some pieces of the pizza to somebody like you. Wait, don't eat it! At any rate, now the pizza uses your money to add toppings and such in order to make it more attractive customers. But the slice that you have, rather than being eaten, will gain toppings as well. Your hope is that eventually the pizza will be worth more money, so that you can sell your slice to someone else and make a profit. And that's really a stock is; a small slice (or "share") of the company!

Ask questions in the comments, and I'll try to improve this as much as I can!