Cruel is a pile based card game in the same genre with games like Shamrocks, La Belle Lucie, and Trefoil. When you play Cruel, it starts with 12 piles of 4 cards each with the Aces dealt into the foundations at the beginning. Like in most solitaire card games, you're trying to build up the foundations from Ace to King by suit. Cruel is pretty strict about the ways you can build on the piles. You can only build the piles down by suit. Like in other pile based card games once all the cards from a pile have been removed, you can't create a new pile in it's place.
Cruel would be an impossible card game if it weren't for the 'Shift' button. Some solitaire games have a 'Shuffle' button that allow you to shuffle up certain cards, but the 'Shift' button doesn't actually shuffle the cards. It does shift all the cards over until they are back into piles of 4 cards each. You can use this button as many times as you like, and since the cards shift in an orderly fashion (unlike a shuffle would) you can predict where the cards will go and use this to shift them into an advantageous position.