These stocks represent part of existing companies. If these companies do well the stockholder will/might receive extra dividends. So if you have stocks of a single company then you will make money in the long run if the company does well (and vice versa). Generally people do not trust a single company, so if you diversify (different strategies apply here) you can make your outcome dependent on a countries economy or the world economy (if the world economy does well, your portfolio will do the same)