Campaigns are a useful tool to track the spending, communications, and results of a marketing campaign. Opportunities, Clients, and Email communications can be stored within a Campaign for easy reporting later on.
If you have not watched the CRM Webinars, please do so before learning about Campaigns.
NOTE: For campaign auditing purposes you cannot remove people from a campaign once it has been created. Email marketing material and campaign reporting are tracked against each person once the campaign is initiated.