When you're creating a campaign, you will need to choose if you want to Target, Exclude, or Ignore an Audience:
Targeting an Audience will tell the system to serve ads to the users in this segment.
Excluding an Audience will tell the system not to serve ads to the users in this segment, even if the user is in a different Audience you are targeting.
Ignoring an Audience will tell the system to pretend this Audience doesn't exist.
It's very important to understand the difference between these settings, specifically Excluding vs. Ignoring an Audience.
In the above example, I'm telling the system to serve ads to the "Cell Phone Browsers" Audience, don't serve ads to anyone in the "Purchased Something" Audience, and to do nothing with the "All Visitors" Audience. Using targeting settings in this manner is a great way to save money with your campaign, as you're not paying to show ads to people who have already purchased something.
You can use this tactic as well by creating multiple Audiences for your site - One for people browsing your site (or maybe even users who put something into their shopping cart!), and another for users who have purchased.