Different Ad Types

There are different ad types provided by Google. Basically, Ads can be divided into 2 main categories:

Ad Units - these are the most commonly seen ads, then can be in pure text or in the form of images. The number of ads in an ad unit base on the size of the ad unit.
Link Units - these are groups of keyword links which is kind of like a site's navigation links. You can choose either 4 or 5 links to be shown for a link unit. When your visitor click on the links, a landing page with the actual ads will be displayed. Those ads are baed on the keywords on the links.

For both category, different sizes (and thus the number of ads/links) are available. You can pick the ones that fit your page's layout:

	Ad Units	Link Units
Horizontal	728 x 90 Leaderboard	728 x 15
	468 x 60 Banner	468 x 15
	234 x 60 Half Banner

Vertical	120 x 600 Skyscraper
	160 x 600 Wide Skyscraper
	120 x 240 Vertical Banner

Square	125 x 125 Button	200 x 90
	300 x 250 Medium rectangle	180 x 90
	250 x 250 Square	160 x 90
	336 x 280 Large rectangle	120 x 90
	180 x 150 Small rectangle

Frankly speaking, it is hard to tell which type works best. It should be on a site by site basis. But in general:

728 x 90 Leaderboard works well as a horizontal banner
160 x 600 Wide Skyscraper works well in the vertical series
336 x 280 Large rectangle works best when embedded inside the content text

It looks like the more space you allocate to the ads, the better is the click performance... seems a very straight forward explanation? If you don't have your site built yet, you probably need to take into account of the ad dimensions when designing your page layouts.