Simple scratchcards require the player to match three (or varying numbers) of the same prize amounts. If this is accomplished, they win that amount. More complicated scratchcards have several different ways to win on one card. Other scratchcards involve matching symbols, pictures or words, or are adaptions of popular (card-)games like Blackjack, Poker or Monopoly.
There are also online versions of the same game which utilize Flash and Java to simulate the experience on a computer.
Scratchcards are a very popular form of gambling due to their low cost. You can also win instantly as opposed to waiting for a drawing like many lotteries.
The low cost to buy a scratchcard is offset by the smaller prizes and lower expected value, compared to casino jackpots or lottery wins. There is a trend towards more expensive scratchcards (20-30USD) that have prizes in the millions of dollars.
The other recent change in scratchcards is the increasing attempts to predict which cards have prizes. In the past, scratchcards have been a random impulse purchase. Now, there are amateur and professional efforts to track the number of prizes won and cards sold so people have access to the current odds on all the games available.