Both expenses and bills are used to record an expense amount but in different scenarios.
You record a bill when your vendor or supplier sends an invoice for the purchase you made. You use bills to keep track of the money you owe your vendors or suppliers.
Examples of when you can record a bill are:
You can record an expense when it is incurred or when the payment is made. You will not be able to pay for it at a later point in time.
Examples of when you can record an expense are:
Books
Online accounting software
for small businesses.