Gambler's ruin describes the idea that any time the house has an advantage in a game of chance, the gambler will eventually lose his entire bankroll. It can also be used to describe the desire gamblers often have of making a final big bet after a streak of bad luck, in the hopes of "winning it all back." When you get right down to it, the idea of gambler's ruin is that if one plays against the casino for long enough, the casino always wins.
In almost all cases, a gambler can be considered to have a limited amount of money to gamble with, while the casino has an effectively limitless amount of money. The casino also has the odds in its favor, sometimes by a very large percentage. Any time a gambler starts playing against the casino, the idea of gambler's ruin describes the fact that a gambler can never take all the casino's money, but the casino can take all of the gambler's money, preventing her from continuing to gamble.
In a standard case of gambler's ruin, imagine that a gambler has 100 dollars. The gambler makes an even-money wager on a bet that gives the casino slightly better odds. Because the odds are in the casino's favor, the gambler will lose more often than he wins. He may win a few times, and have more money than he started with, but because the odds favor the casino, in the long run he will lose more often than he wins. Because the casino has unlimited funds, and the gambler has only 100 dollars, he will eventually run out and be unable to make another bet.
In the other sense, gambler's ruin describes the desire to try and win big, by making a large bet when the gambler has almost exhausted her gambling bankroll. The gambler makes a series of small bets, and over time loses money, since the casino has the advantage. When she realizes that she has very little money left, gambler's ruin describes her desire to try and win it all back, rather than accepting the loss and walking away with what money she has left. No matter how strongly the gambler wants to win, if she keeps betting, the casino will keep winning.
Gambler's ruin applies to any game of chance in which the house has an advantage over the gambler. This includes any casino game played against the casino, such as blackjack, craps, roulette, Keno, and slot machines. The only way to avoid gambler's ruin is to accept that fact that you can't gamble to win, so you should only gamble for the enjoyment of the game. If you play long enough, the casino always wins.