Here is a thorough key:
Hit: each time one of your widgets is visualized anywhere.
Click: Each time anyone clicks on "Buy" on one of your widgets, it generates a "click".
Revenue: Each time anyone purchases a product from one of your widgets it generates a "revenue" based on the purchased product's price and the persentage based by StreetLib Sell (15%).
Order: A purchase made by someone from one of your widgets is an order.
Volume: The number of orders made.
Value: The total paid by the purchaser for each order (for the ebook, it's the cover price), from which is calculated your revenue.