This occurs when you switch from paying in full to carrying a balance from month to month. Most card issuers charge interest from the day a charge is posted to your account if you don't pay your account in full each month. If your credit card issuer is also a bank where you have money on deposit, you may have signed an agreement when you opened the deposit account which permits the bank to take those deposited funds if you become delinquent on your credit card account. If you sign up for a credit card with a low introductory rate, when the low rate period ends your existing balance will likely be subject to the regular, substantially higher interest rate. Many issuers offer extra benefits to encourage you to sign up for their card such as lifetime warranty on purchases made with their card, various discounts, or protection if an item purchased with their card is lost or stolen.
Many card issuers impose both finance charges and transaction fees for cash advances.
Many card issuers offer debit cards that deduct money directly from your checking account. Check out The Credit Improvement Package for more information like this.