A bill is an invoice you send to someone else, to request payment. An expense is an invoice someone sends to you for payment. This “invoice” might be as simple

A bill is an invoice you send to someone else, to request payment. An expense is an invoice someone sends to you for payment. This “invoice” might be as simple as a docket from a cash register. If you receive an invoice from someone, it can be classified as an expense if it’s legitimately part of your business - depending on local law. If it is an expense then it’s deducted from your income so that you don’t get taxed on it - you should only be taxed on profit, not revenue. If it’s not a legitimate business expense then you can’t deduct it from your income and the tax you pay is higher.