Banner ads come in all shapes, sizes, and types. You've probably
seen them running along the top or side of web pages. For an
example of how a banner ad fits into an average web page, see an
example at http://www.howstuffworks.com/banner-ad.htm
But it can also be helpful to see several side-by-side to see
what makes a well-designed ad effective. For examples of static and
animated banner ads, you can see:
http://www.adcherry.com/custom/banner_ad_portfolio.php or your
favorite website.
For examples of banner ad sizes, check out the IAB's website at
http://www.iab.net/iab_products_and_industry_services/1421/1443/1452
I hope this has been helpful.