ABA number explanation

A unique code present on a check assigned by the American Bankers Association (ABA) that identifies a particular bank or finance institution issuing the check. This code is primarily used by US-based financial institutions, who own accounts at the Federal Reserve Bank, to process check transactions including deposits and clearances. For example, Bank of America in Northern California is assigned the number 121000358 in case of any paper check transactions. Also called check routing number, ABA number.

A link to the Federal Reserve Board to find ABA numbers:

http://www.fededirectory.frb.org/search.cfm