What is the purpose of a debit memo

I am not a bookkeeper and thankfully do not have to balance my cheque book any longer, because of on line banking, but I dont understand why, as a bookkeeper you dont know what a debit memo is.

Here is how EHow explains the purpose of debit memos:

Debit memos, or debit memorandums, are documents used in business transactions. Debit memos show that either a transaction has already occurred or will occur. They are often used on bank statements and on accounts payable invoices as a bookkeeping device.

What is a Debit Memo?

Debit memos function like invoices. They keep track of money or fees owed, as well as the running balance on an account, for example, when a company fails to pay a bill, or only pays part of the money it owes. In banking, debit memos serve to keep track of money owed on an account. In banking, an account balance is a liability for the bank--the balance is the money the bank owes the account holder. So, when the balance is decreased, for example, when the account holder writes a check or owes the bank a late fee, the bank's liability is decreased. The bank notes this reduction with a debit memo.