The double entry system of accounting or bookkeeping means that every business transaction will involve two accounts (or more). For example, when a company borrows money from its bank, the company's Cash account will increase and its liability account Loans Payable will increase. If a company pays $200 for an advertisement, its Cash account will decrease and its account Advertising Expense will increase.
Double entry also allows for the accounting equation (assets = liabilities + owner's equity) to always be in balance. In our example involving Advertising Expense, the accounting equation remained in balance because expenses cause owner's equity to decrease. In that example, the asset Cash decreased and the owner's capital account within owner's equity also decreased.
A third aspect of double entry is that the amounts entered into the general ledger accounts as debits must be equal to the amounts entered as credits.