What is the difference between Top Grossing and Top Paid in the App Store?
There are three categories: Top Free, Top Paid, and Top Grossing.

In the Top Free category, the apps with the most downloads that have a price of $0 make the list.

In the Top Paid category, the apps with the most downloads that have a price greater than $0 make the list.

The Top Grossing category lists the apps with the highest total revenue (that is, price * quantity sold + revenue from in-app purchases). A $50 app is likely to have fewer downloads than at $0.99 app. The Top Paid list will likely include many inexpensive apps that get purchased often. The Top Grossing category is to indicate which apps have the highest total spent on them.