Leaderboard : 728x90 pixels
Slightly larger than the standard Banners, designed to stretch the full width across the top of a webpage.

Banner : 468x60 pixels
This is the banner format that you probably know the best,
and is the original banners format.

SkyScraper : 120x600 pixels
Ideal for those who have a column-based design. A very eyecatching size, and if well designed, your SkyScraper will attract greater clickthrus than the other ad formats.

Button : 120x60 pixels
If space is tight on your website then this is an ideal format to use. You are allowed to embed two buttons on your page at any one time.

Half Banner : 234x60 pixels
As the name suggests, it's half the size of a standard banner.

Rectangle : 300x250 pixels
An increasingly popular size for advertisers, because they're easy to spot, but they also take up a lot of webpage space.