Affiliate Marketing Secrets

I’d be interested in buying and selling shares during my summer break in order to make some extra cash when i’m not working, perhaps start doing it to earn serious cash in the future. My very crude and basic view of how it works is that a buyer buys shares for a certain amount, waits for them to rise in value, and then sells them, cashing in on his efforts. It seems pretty simple from this and easy to do, but if it were everybody would be doing it.
Questions i have relating to this are: How does the buyer earn money, who pays him or her? And what is a portfolio?
Thanks, i tried reading about it online but found it a little complex. I figured talking to real people would help.