All the numbers written across a check mean something to the bank that the check is drawn on, and to other banks who process the check. These numbers are all standardized in the United States for ease of processing. So no matter where you are, the bank can tell by the numbers on the check what bank the funds are drawn on, the account number and even your check number. Here's how to find those numbers for yourself.
Look at the bottom of the check and you will see three sets of numbers. The first set of numbers has a symbol around it that looks like a sideways face with one short line and two vertical dots to the left of the short line. The number between the two symbols is the bank routing number. This is how banks know what bank the check is drawn on. This number is always nine digits in the United States.
The second set of numbers is the account number. The account number is followed by a symbol that looks like two short side-by-side vertical lines with a large dot to the upper right of the two short lines.
Note that there is one number left; that matches the number in the upper right corner of the check. This is the check number. The two numbers should always match.
You may have a debit card, but it most likely does not have this account number on it, although it is tied to your checking account internally.
You may also find your account number on your bank statement and on the papers that you received when you opened the account.
Always keep your checks and checkbook out of sight, which is the first step in protecting the funds in your account.