Generally, Campaigns will have one of the following primary goals:
Lead generation
Lead generation campaigns such as direct mail, email blasts, seminars, conferences, and roadshows. Lead generation campaigns directly generate new prospects. In Salesforce, you can easily track the effectiveness of each campaign in terms of the revenue generated in new business.
Brand building
Brand-building campaigns such as print ads, billboards, and radio advertisements. Brand-building campaigns may not generate direct responses and make the calculation of campaign ROI not straightforward.
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Also, watch this video on Campaigns in Salesforce: