There are two options which Google's pay per Click Marketing offers.
It is possible to advertise in website content or advertise in search results. It is also possible to advertise in both sections.
Content advertising means that your ads come up on sites which include Google's "Adsense".
When you set your ads on search results they come up in Google's search results.
Nowadays Adsense is so huge that millions of Webmasters have implemented Adsense on their websites.
But there are both, positive and negative, factors on both advertising methods.
When you advertise in Google's search results it is very probable that only people see your result who actually are seeking that what you promote. The problem is that there are people out there who just click on your ad's to bring your ROI down. And you as an advertiser pay for every click which is made on your ad. So it is very easy for a competitor to blow up your campaigns just with clicking on your ads.
Of course this sabotaging is also possible when you advertise in content network. But there's also a very positive factor on content advertising. Only people who are interested and who will to buy your product will see your ads.

But pay per click Marketing requires also a lot of knowledge about writing the right ads. It's the most important thing to write the right text on your ads. Let me give you an example:
Someone searches for dog houses. You have a website which sells only dog food.
So you never may write something like "Everything about your favorite pet - the dog"
So the searcher would probably click on your ad in a care free manner. But he is searching for dog houses but sell food. So he leaves your site.
So think of how to write your ads correctly! It is very important otherwise you will throw to much money in the wind!