If you're establishing direct deposit through your employer, ordering checks, or transferring money between financial institutions, you'll be asked to provide a routing number along with your personal account number to execute the transaction.
A routing number is a 9-digit code identifying a financial institution, such as a credit union, bank, or investment firm. This number helps reduce confusion between financial institutions that may have similar names and allows money to flow between these institutions without glitches. Sometimes an institution may have more than one routing number depending on its size and the type of account (checking account vs. investment account, for example).