When you're paying for campaigns (AdWords and other types), you want to know how well they're working: how many visitors they're bringing in, how much money those visitors are spending.
If you find that a campaign is bringing in lots of visitors, but a lot of them are leaving after viewing only one page (bouncing) or viewing a number of pages but not converting, you may have a problem with the landing page associated with that campaign, or with the pages linked from the landing page. There may be jarring graphic inconsistencies between your campaign ads and the landing page to which those ads direct traffic, or the messaging or content of the landing page and linked pages may not be consistent with your campaign. You may be running a very specific campaign but rather than delivering visitors directly to the page for that product, you're delivering them to your home page and forcing them to wander your site in search of the specific bit of information for which they came.
If you're running the same campaign through multiple sources (e.g., Google, Yahoo, Bing), you can view the data by Campaign, and add the secondary dimension Source so you can compare the results from the different sources side by side. If you find low productivity from a particular source, you can try a different campaign there, or redistribute your campaign spending to the more profitable sources.
Because the Campaigns report includes all visits (from campaigns and from other sources), you may see a number of visits where the campaign is listed as (not set). The majority of these are visits that have no campaign tagging. Some of these may be from AdWords campaigns that have some error in the tagging (see AdWords Campaigns for more information). You can eliminate these statistics by using the Search option above the table, and setting the parameters to Exclude Campaign Exactly matching (not set).