AdSense publishers can choose from a large number of ad layouts for displaying advertisements on their sites. The layouts vary greatly in type and format and can be customized to match (or contrast with) the colors of your site.
There are two basic types of AdSense advertisements: ad units and link units. Ad units are conventional pay-per-click advertising blocks. An ad unit can display either a sequence of text ads or (for some formats) a clickable image ad. Link units, on the other hand, display links to pages of pay-per-click advertisements. (With a link unit, two clicks are required to earn you money because the first click just opens a page of ads.)
For ad units, you can choose from a dozen different ad formats. Each format has a name, but is described primarily by its physical dimensions in pixels and the number of text ads it displays. For example, the leaderboard is a block 728 pixels wide by 90 pixels high that displays four text ads, whereas the button displays a single text ad in a 125-pixel square.
Link units have a smaller selection of formats to choose from, described by their dimensions on the page and the number of links (currently either four or five) they display.
Ad units are much more popular than link units. Which format works best depends greatly on where the ads are placed on the page. You can place up to three ad units on a single page, but only one link unit—for a maximum of no more than four units in total. Don't go overboard with ad units, however. The old adage "less is more" applies to AdSense: Remember that the ads are generally ordered in terms of descending payment values, meaning that the first ads shown usually pay the most. The fewer ads you show, the higher your per-click earnings are going to be.