Web-based advertisements should be used sparingly, as having too many ads can make a site feel less informational and more promotional, which tends to result in a loss of interest from your average user.
Ads should not be invasive, and if they are only going to be placed on certain sites, their colors should not be disruptive to the overall site design, but should still stand out.
Ads should be as small in file size as possible, specifically below 75k, so that they load quickly for low bandwidth users (still the majority in the USA).
Ads are typically GIFs or JPEGs.
Banner & Button-type ads tend to be animated GIFs.
Amazon-type ads tend to be static, and sometimes have actual (html) text in them, above or below smaller images.
If possible, ads should be on a rotator or aggregation system (typically javascript based). This gives users the impression that, when they return to the site, things are updated (even if the content is the same).
Average user's screen resolution: 1024 x 768. Average viewable area in a web browser @ 1024 x 768 resolution: ~990 x 570
Common Ad Styles
Typical banner ad. Common, well known and used all over.
This is based on the Amazon.com "sidebar" style advertisements. Very popular as it takes up little horizontal space and can be placed in unused margins on the sides of sites.
Popular on sites that tend to scatter ads around their pages. Often used in multi-site aggregators.
Photo gallery thumbnail-sized. Not commonly used, but can make for a nice content-area ad.
SAMHSA Bucket-sized ad. Easily placed on the SAMHSA.gov homepage and all over sites using the current SAMHSA.gov template. Examples:
