Pre-paid calling cards are essentially long distance accounts that you pay for before actually using them. Each time you make a phone call, the appropriate amount of money will be deducted from your account balance. The amount will depend on the destination you call and the amount of time you speak. Once you have used up all the money in your account, the card will no longer work.
Every card is different, but each works on a similar principle.
Simply dial the toll-free number and follow the voice prompts. At some point, you will be required to enter your PIN number. It is usually bought on-line or over the phone. Unfortunately there are many. First of all, many of the companies who offer attractive rates to one country may offer unattractive rates to others.