Hello
Google allow you to sign up for their advertising network program which is called ‘Google Ad sense‘ which gives you the ability to choose different styles of advertisements to display on your own website or blog, which are relevant to the content on your site, and Google pay you a commission when one of the ads is clicked on for hosting the ad.
Through the Google Ad sense control panel you can choose the following types of ads:
Text Ads – purely text based ads, these tend to blend well with content, and don't appear to be blatant advertising
Image Ads – much more obvious that they are an advertisement, usually animated image banners
Link Unit – this is a list of key words, which then takes the user to another screen with text ads, requiring 2 clicks to earn your commission


From my experience I have found that the text and image ads convert the best, but I have found while image ads may have a higher conversion rate than text ads (differing with your placement), text ads are often higher paying per click.
Depending on the purpose behind your site, your style, and your readership its always worth doing some testing with the various ad groups before you settle on your decision of which to use.