Which Type of Online Ad Is Most Effective?

From banner ads to pop-ups, text ads to interstitials, your choices when it comes to online advertising are daunting, especially given the range of pricing that exists. The trick is deciding which type is best for your business.
Unfortunately, there isn't much consensus on which type of ad works best, in part because it depends upon the product, the host site, the user's preferences and mood, and other variables. However, there are some general guidelines based on industry testing and research that are worth considering.

The online advertising terrain is constantly shifting as advertisers try to stay one step ahead of user ad rejection. Many ad types, such as pop-ups and interstitials (ads that appear briefly while you wait for a page to load), can negatively affect the user experience and therefore the advertising's effectiveness. Some advertisers seem stuck in the broadcast model and may never understand that the best way to alienate a potential customer is to hijack their browser with an ad they can't control or didn't ask to view. For this simple reason it's highly advisable that you only elect inline ad campaigns.
Studies show that animated ads, pop-up ads, and interstitial ads have the highest visibility, but also tend to have the lowest click-through rates. Banner ads, and to a growing extent text ads, are subject to what the industry is calling "banner blindness." In other words, having grown so accustomed to their presence, users have stopped seeing them. Likewise, animated ads have an early point of diminishing returns, because with too much animation visitors quickly tune them out.
Simple text ads, such as those served by AdBrite and Google AdSense, tend to be the happy medium when it comes to cost vs. click-through rate. Other research indicates that up to 85 percent of ad conversions can occur as late as two weeks after the ad was viewed.
The implications are that advertising isn't always the integrated process many suppose it to be, and that users are attuned to more than just an ad's presence. The general consensus seems to be that some types of ads — pop-ups, interstitials, and banners — are good for branding, while text ads are best for overall conversions.

Source: http://www.allbusiness.com