You set a daily budget for each campaign for your ads to be displayed. The amount you enter is the average daily amount you are willing to spend over a calendar month. For example, if you set your daily budget at $100, you may receive up to $3,100 worth of clicks in that calendar month (assuming a full 31 day month). At the end of the month the average daily value of your advertising spend will not exceed your desired daily spend. However, on any given day you can spend less or more than your actual daily budget. For example, if your budget is $100 and you receive $50 worth of clicks on the first day of the month, you may receive up to $150 worth of clicks on the second day of the month. This would bring your total spend over two days to $200, which is in line with your average daily budget.