Credit Card accounts are used to keep track of your current balance on a single credit card.

The balance on a credit card account represents your current debt on the credit card.  

Therefore a credit card account will usually have a negative balance.

You can setup a credit card account as you would any other account, by navigating to Account > New Account.

A credit card account can store information such as card number, expiration date, interest rate (labeled APR), and bank name for your convenience. However, these fields are not required.
None of these values, including the interest rate, effect the operation of the account.

Rather than the "Initial Balance" field, you'll see "Initial Debt".

Credit Card accounts usually have negative balances.

This account type generally represents money that you owe and has a negative effect on your net worth.

Entering Credit Card Transactions - 

Purchase transactions are entered into a credit card account just as they are in a bank account - with a description, category, amount, etc.

Purchases should be entered as 'charges' that decrease the account balance (and represent the increase of debt).

Credit card payments are recorded as a transfer of funds from a bank (often a checking) account to the credit card account.

This reduces the balance of the checking account and reduces the debt on the credit card.