The Campaigns tab shows you the open, click, and conversion data for each sent campaign. The numbers you see here are based on unique people opening, clicking, purchasing, etc.
When you click into an individual sent campaign, you will initially see the overview report for the campaign with high-level performance metrics displayed.
The core open/click data you see displayed in the middle section of this report represent unique opens and clicks -- these numbers will match the numbers you see on the Campaigns tab front page. At the bottom of this report, however, you'll see there is also data around Total Opens and Total Clicks.
These totals represent all opens and clicks recorded. For example, if one person received your campaign and opened twice, we would count two total opens. These totals will thus typically be higher than the unique counts. If you see total open numbers that are significantly higher than unique opens, this could signal that recipients are consistently forwarding your emails to others.