Catalina coupons are coupons that print out at the register after you make a purchase. A portion of what prints are advertisements for future purchases or coupons. However, when I refer to a "Catalina," I am talking about something different, which is like cash off your next purchase.
A Catalina is triggered to print when you purchase specific items. For instance, if you buy 3 boxes of XYZ, you will receive a $2 Catalina. If you are purchasing 6 boxes, it will only print one. You'll need the purchase rung up separately if you want two. If the cashier lets you (almost always do) you can use the one you just earned for the second transaction. So, Catalina coupons "roll," unlike Register Rewards at Walgreens.
Catalina coupons can be used for cash off a future purchase at the store the coupon was generated at. (You can purchase almost anything, though there are a few exclusions listed on the coupons.)
Basically, you hand them over to the cashier and the amount you earn is deducted from your next purchase. No change will be given, so make sure your total exceeds the amount of the Catalina, after your other coupons are deducted. They have expiration dates on them, so watch the dates.