It can either be "fixed" or "variable". Fixed-rate APR-s are usually a little higher, but you know exactly how much you will be charged each month. A variable or floating rate will fluctuate based on a published index.

For example: The APR on your credit card is 18%. Typically, credit card companies calculate that rate on either a daily or monthly basis. The daily rate is calculated on your daily balance and the monthly rate on your monthly.
Let's say you charge $800 for a new TV. Few weeks later, you get your bill and you decide to pay only the minimum. This is when finance charge steps in, and if your card works on a monthly basis, you will be charged 1-1/2% , (or1.5 %) interest on $800.

Where did the 1.5 % come from? That’s 18% divided by 12, as 12 months in a year. (Annual Percentage Rate).

Next month, when you get your bill, and you did not charged anything on your card, and you made your minimum payment of $15, you will be unsatisfised.

From the $15 minimum payment you made, $12 pays the interest and only $3 will reduce the $800 down to $797. — which does not seem like much progress.

This is where you have to be carefull. If you only make the minimum payment each month, most of your money goes toward paying off the finance charge—the amount you’re being charged on the money you’ve effectively borrowed from the credit card company.

So, if you borrow the money for a long time, you would end up paying far more for the product than you originally wanted.

This is why it’s far more smart business—to choose a card with low APR and pay not only the minimum payment but as much as you can !