There are two ways you can view the aggregate open rate statistics for email campaigns.
The downloadable report is a .csv comma delimited file that can be opened in other software programs (i.e. MS Excel) for further analysis. The file contains the details of each email campaign including the send date, subject line, total number of views, unsubscribes and open rate.
For aggregate open rate statistics for email campaigns sent with email automation, you have more options. View related FAQ: How do I view aggregate statistics for automated email campaigns?