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.
They can be great sources of revenues, driving the web and the internet to become a more crowded place with packed with variety of information as we know them.
There are generally two types of external links. The first is organic link that is brought from natural reasons. Like for example when people like a post you've shared and want to share to their own audience. The second is advertisements and paid links, and these are different. They happen not because they like you. They happen because they're purchased for a reason.
The thing about advertisements on the web and paid links, is that not properly displaying them is not at all wise. Especially if you wish to rank well on search engines.
If you do not follow this guideline, you website will likely to lose ranking on search engines.
Knowing how search engines work, they use links as one of the important ways to rank web pages. But as advertisements and paid links started to become popular, they started to mess things up because they make things more difficult for search engines to determine whether a link was a natural link, or a paid link, created to advertise something for a price.
At that time, many people took advantage. They bought a while lot of links by paying other websites to have their links on their web pages. What happened afterwards, those links manipulated Google and other search engines in ranking websites.
Low-quality contents became popular due to the massive amount of paid links they have, burying high-quality contents that struggled organically. Their motivation was not attracting users to their pages, it was to get more links to make their own website rank higher.
Manipulation to trick crawlers are prohibited by all search engines. As a result, search engines that acknowledged the scheme started to revise their algorithms, putting paid links that were once wonders, down. Those websites that weren't displaying paid links and advertisements properly were losing their ranking, and some even have themselves removed from search engines' index.
But anyhow, it's not against any rules to put advertising or paid links on your website. What you need to do, is just follow the guidelines.
The guidelines are made so websites owners don't manipulate search engines' crawlers. This is to give the link so those crawlers will know the difference between ordinary link and advertising/paid link. Any advertisement or paid links that pass rank are against the Google guidelines.
To do this, any paid links or advertisement should have a nofollow attribute ("rel=nofollow").
Advertisements from Google and other search engines won't affect your rank because they are all nofollow What you need to take care of, are those advertisements and paid links that you put on your own, like for example: direct ads.
If you are selling paid links or you are displaying ads from individual companies or websites you need take action by making those links nofollow. The same goes with affiliate links. Because they are also regarded as ads, they also should be nofollowed.
Another way is to redirect those links to a a file that is excluded by the website's robots.txt
This strategy is more for advanced webmasters because it requires them to take the advertiser's link and send it through a redirect via a page on their website which is blocked by their robots.txt file. This method still allows people to go through the link, but the link won't pass any rank, so it won't affect the advertisers' or the webmasters' web ranking.
Usually, this strategy uses a folder that is blocked by the website's robots.txt. Then inside this folder, the webmaster create one page for each advertiser. And on that page, there is a simple redirect.