different ads

Text Ads

These ads have only text and an URL as the content. These are the most popular among advertisers as it usually fits pretty well with most websites including blogs. They are also easy to create for most small advertisers and has a fast approval time as the approval process is automated. 

As these ads contains only text. it is very easy to customize the font colors and the overall look and feel of the ads to better suit your website. The ads are static and non-intrusive in nature, which means it does not steal focus from your own website and website content

Image Ads or Standard Display Ads

These are graphic ads which is a static or animated image in a predefined size. These ad type takes more time and effort from the advertisers, and also require to approved by Google before it is shown in the search network.

These are animated or static ads that can be developed in any of the technologies, including HTML animated GIF or flash. Some of the larger ad sizes also support video ads.
Just like the text ads, these have only a single interaction. clicking anywhere on the ads takes the user to a destination webpage. 

Rich Media Ads

These are usually animated ads developed using flash which also allows the user to interact with it in many different ways. The users can expand the ads to a larger size, initiate a phone call, play videos among otherthings. These type of ads are much more complex to create and needs significant time and effort to make sure that all interactions works well across platforms and web browsers.
These ads provide a much richer experience for the user, but at the risk of annoying some users. Because of the complexity and effort involved, these are developed by mostly big budget advertisers.

Link Unit Ads

These are ads that display a bunch of topics that are relevant to the user or your webpage content. clicking on any of these topics, opens a page of related ads relevant to topic that was clicked. These ads are highly relevantto the topic and hence have a high CTR. Any click on these ads on the page will directly be attributed to your website and you earn revenue from those.
These type of ads display similar to the text ads. On the flip size. it requires at least two clicks from the user for you to get any revenue.