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.