A Google AdWords account are divided into three levels, they are account level, campaign level, and ad group level.

At the first level, your account level, your account gets associated with your unique email address, password and billing information.

At the next level, the campaign level, you can setup your daily budget, Google Network distribution preferences, geographic and language targeting as well as campaign end dates.

At the third level, ad group level, you can create your ads and choose your keywords. You can then set bids for your ad groups or individual keywords or sites. Within each of your ad groups you can create one or more ads and select a set of keywords to trigger those ads. When you create multiple ads for you ad group they will be rotated by Google.