It is a Delta Skymiles credit card from AMEX. You can use it per se like a debit card, subtracting the amount in your checkbook everytime you use it. Every month the bill comes in and you pay it. I use it for things I would normally use my debit card or a check for. The miles rack up fast.
For instance, I have 2 savings accounts at my credit union. One is for vacations, and one for Christmas. I charge my vacation plans with the skymiles card, and when the bill comes in, I take the amount out of my savings to pay it. Same for Christmas gifts.
Never carry a balance with this card. Like Jillpie says, use it as you would a debit card. I get double miles grocery shopping at Stop & Shop, 5000 miles signing with MCI long distance for 5 months. If you have money set aside for a big purchase, buy it with the card and pay it off with the money you saved when the bill comes in.
I already have 20,000 miles in the last 6 months of having this card. My next flight will be free.....