If you have recently  joined adsense program, and might wonder, which add unit should be used then here is the answer. If you have implemented Adsense for content then you will see plain text ads. And If you implemented adsense for content, and select to show text and image ads as well, You will see both test and image ads. You can even display just text or image ads. However, I will recommend you to display both text and image ads.

Text Ads

These are the standard adsense ads and most popular one. It contains a text ad that displays a short headline, ad text usually a description  and a URL of the advertiser website or blog. Text ads work best for the website which is content oriented. If your website too have lots of content it will be wise to use a text image. Since the Google ads are contextual it shows the ads which are related to your web content. It shows different ads as per reader location, and performs really well.

Image Ads

Image ads are the one that shows graphical content. It can be a plain image or the animated one. Image ads are also contextual and show ads related to your website content. Consider using image ads when your website has been less content. And it also performs well on a site which is an e-commerce related. Image ads appear to be the website product and can produce more clicks.

Quick Tip: Different ad format works for different website test all the formats and finds out, which work best for you.