Running Pay-Per-Click ads on Google is like going to the market and buying fruit. Some are good, some are bad. You're constantly testing and tasting to see which keywords and ad combinations bring the best results.
And that's not an exaggeration–I mean, you literally plonk your ad in the center of a crowded marketplace full of people searching keywords related to your business. And you just pay Google a small fee for every time a customer responds to your ad.