If you do not have overdraft protection, the full amount you are overdrawn is typically transferred out of your checking account the next time a deposit is made.
If you try to use your debit card when there is not enough money in your account to cover the transaction and your account does not allow overdrawing, the transaction will be declined. The bank will also charge an NSF fee to your account.
Some people may think that the traditional fees associated with overdrawing are too expensive for the convenience provided but feel it is worth it if they are able to have lower fees with overdraft protection. While it is best to manage your finances so that you never need to make a transaction when there is not enough money in your checking account, having the ability to overdraw, especially when used in conjunction with overdraft protection, can be a helpful tool for many people.
Retailers also use check verification services to find out if they should accept your check at the checkout counter before they let you walk out with merchandise. But they may overdraft their account so you want to notify them as quickly as possible about what happened. The legal consequences, again, depend on state law.