Quite simply there are 5 account types and each one can go up or down as follows.
D E A D C L I C This acronym stands for Debit Expenses, Assets and Drawings, and Credit Liabilities, Income and Capital.
Account Group
Account Value Increase
Account Value Decrease
Balance
Assets
A/R
Cash
Contra Liability
DR
CR
DR
Liabilities
Deposits
Taxes Due
Prepaid Sales
Contra Asset
CR
DR
CR
Owner (Stockholders') Equity
CR
DR
CR
Owner's Drawing or Dividends Account
DR
CR
DR
Revenue (Income)
Sales
Gains
CR
DR
CR
Expenses
Discounts
Losses
DR
CR
DR
You apply this DEAD CLIC rule if an account goes up in value. If an account goes down in value, you apply the opposite. In other words, if an expense increases in value, then you debit the account (because the DEAD CLIC rule says to Debit Expenses). If an expense decreases in value, then you credit the account.
Remember also that every financial transaction affects two accounts, one is a debit, the other a credit. This is why we call it double entry (not single entry) bookkeeping. Refer to the T-account examples below, and on each Business Transaction page.