Advertisements are all over the web. There are usual and in fact a normal way of things on the internet. It's not against any search engines' policies or against any rules for that matter.
Advertisements and paid links are the life blood of the web. Advertisements and paid links are those that you can put on your page that point to another page, owned or controlled by the person who is paying them. That link can be an image and/or text, or anything others that will link your page to another.