Let's first talk about stocks or equities It's easiest to understand stocks if you imagine going into business with some of your friends. For example, let's say there are four of you starting a business together, and you each put in exactly the same money to get started. You agree to work on it together and evenly split whatever profit you make. Each of you owns 1/4 of the business, so you get 1/4 of the profit; this is a pretty simple idea.

A "share of stock" is just a way of keeping track of who owns a part of the business, and what amount. Since there are four friends owning the business equally, you could say each of you owns one share, with a total of 4 shares outstanding (the total number of shares everybody owns). Or you could also say that each of you owns 2 shares, with 8 shares outstanding.
Or that each of you owns 100 shares, with 400 shares outstanding. What's important is the fraction of the outstanding shares that you own. That's the portion of the company that's yours, and the share of profit you're entitled to. Really the whole idea of a "share" is just record-keeping, to track who owns what fraction of the business. When it's time to hand out the profit from the business, it will be evenly divided amongst all the outstanding shares.

Now you may decide you're tired of being in business with your other 3 friends, and you want to quit. But the others want to keep going -- they aren't ready to quit. What do you do? You find someone else who wants to take over your part of the business. Once he takes over your shares then he (not you) gets a 1/4 share of the profit, so he should be willing to pay you something for it. The question is, how much will he pay you? It all depends -- on how much profit the company is making, how much he thinks profit will change in the future, and so on. There's no simple way to judge the value of it, so the price you sell your share to him will depend on whatever the two of you agree on.

Now The "stock market" is just an efficient way that people who own shares of businesses can find people who want to buy their shares. And when you sell there is an auction to see who will pay the most for that share -- so you know you're getting a fair price.
It all works so well that you can scale it up to businesses that are so large they are owned by millions of people. Most of them don't even know each other! But they still get their share of the profit, and can sell their shares to somebody else when they want to quit.

1) Stocks give you partial ownership of a company
2) Not quite sure what you mean. It impacts you because you invested your money with the company. You want the company to do well
3) Sell the stock for a higher price than you bought it for.
4) A stock's price is based on expectations of future profits.