A split test allows you to send different versions of the same email campaign to see which gets the better open and click-thru rates.
Example: you want to send an email to 1000 people, but you're curious about which subject line would be the most effective. With split tests, the application will divide the mailing up randomly to 500 people each and see which one performs better, based on the number of opens or links clicked in the email, etc. In this case, the application sends to everyone and shows you the results.
The second way you can run split tests is to choose a percentage of your list size to send the test to. Simply choose which percentage of the list should get the test, what criteria determines a winner and what time the remaining lists should get the winner.
In all cases, both tests display like normal reports with full opens, clicks, etc. These can be found under Reports/Split Test Reports.
Split Tests fire out 20 minutes after scheduling.