Total Output Volume
The total value of all transaction outputs per day. This includes coins which were returned to the sender as change.
Estimated Transaction Volume
Similar to the total output volume with the addition of an algorithm which attempts to remove change from the total value. This may be a more accurate reflection of the true transaction volume.
Bitcoin Days Destroyed Cumulative
Bitcoin Days Destroyed is a measure of the transaction volume of Bitcoin. If someone has 100 BTC that they received a week ago and they spend it then 700 bitcoin days have been destroyed. If they take those 100BTC and send them to several addresses and then spend them then although the total transaction volume could be arbitrarily large the number of bitcoin days destroyed is still 700.