Top ads are ads that have been upgraded to also appear in the "Top ads" section at the top of a category listing page for several days. More people read and reply to these Ads because of their prominent position on the first page of a category over a long time period. Simply put, buying a Top ad is the best way to help sell your item.
Note that if more than 5 Top ads are purchased in a category, only a random five will be shown. The ads which are randomly picked will change each time you look at the page.