Difference between campaign budget and ad set budget?

Campaign budgets automatically adjust ad budgets across ad groups based on their performance. While ad set budget is manual, where you have control in deciding how much each ad set gets.

For example; if you have a budget of $40 and you have 4 ad sets:

1. In campaign budget, the money will be distributed based on the best performing ads. So one ad set may get $15, $10, $8, and $7

2. In ad set budget, the money will be distributed evenly where each will get $10 unless you make adjustments.