That magnetic strip is made up of a whole lot of minute iron-based particles that are on a strip of magnetic material, and the way it works is that your data is stored by changing the magnetism of those particles. Those are the types of card available. HiCo cards are long-lasting for frequent use, and LoCo cards are better used for either infrequent use, or for uses where the data is often changed on the card, like the gift cards you get in supermarkets. If you want the technical bit, HiCo is short for high coercivity, and conversely, LoCo is short for low coercivity. Coercivity is simply how resistant the particles on the magnetic strip are to being demagnetised, or to put it another way, how strong the magnetic strip is. Your credit card is probably around Oe or upwards.