So what is the difference between a slot machine and an online scratch card?

Betting machines work with a random number generator (RNG). An RNG is a computer that generates random numbers. Each number generated by the RNG corresponds to a specific combination on the reels. The computer in the slot machine ensures that that particular combination is displayed. A scratch card does not work on the basis of an RNG, but the prize pool of a series of scratch cards is drawn beforehand. So your chances of winning a prize are also not the same with each scratch card you play.