If you are creating banners for your site, you can create whichever shapes/sizes you wish. But, if you want to put advertisements up supplied by a place like Google’s Adsense, you want to probably follow some sizes that are supported (all listed in pixels and all width x height from Google Adsense as of December 7, 2013):
Most used
- 300×250 (medium rectangle)
- 336×280 (large rectangle)
- 728×90 (leaderboard)
- 160×600 (wide skyscraper)
- 320×50 (mobile banner)
Horizontal
- 970×90 (large leaderboard)
- 468×60 (banner)
- 320×100 (large mobile banner)
- 234×60 (half banner)
Vertical
- 120×600 (skyscraper)
- 120×240 (vertical banner)
- 300×600 (large skyscraper)
Square
- 250×250 (square)
- 200×200 (small square)
- 180×150 (small rectangle)
- 125×125 (button)
Link Units
- 728×15 (horizontal large)
- 468×15 (horizontal medium)
- 200×90 (vertical x-large)
- 180×90 (vertical large)
- 160×90 (vertical medium)
- 120×90 (vertical small)
Wikipedia has a nice visual and a list of the more popular sizes here: http://en.wikipedia.org/wiki/Web_banner
Stick with the more popular sizes and your site can probably easily support AdSense, Amazon, and other ad-serving companies.