Pay Per Click advertising — PPC for short — can be an excellent method of getting your message in front of potential customers. PPC ads are those ads you see on the search results pages of Google, Yahoo, and other websites. You may not have noticed them — the ones marked "Sponsored". If an ad says "Sponsored", it's a paid advertisement, not a regular search result.
As an example, here's a screenshot of PPC ads at Google. Notice the pink box at the top, and the entire right-hand column … they're all PPC ads.
Now, you can do this too, by signing up for an account at Google, Yahoo, etc., noting which keyword phrases you'd like your website to be found for, writing your ads … and that's about the time you discover that you'll be bidding against competitors for your ad placements. There are other issues as well — and, while of course you'd like the top position, your competitor(s) may be driving the bidding price beyond what is feasible for you, or simply what you'd like to pay. And thus you may find your advertising dollars draining from your account without doing you as much good as they should.