This game is a variation of match-three games such as Bejeweled. Each level has a game board filled with differently colored candies, and might contain obstacles. The basic move of this game is horizontally or vertically swapping the positions of two adjacent candies, to create sets of three (or more) candies of the same color.
This game is separated into levels, and each level has its own requirements to complete. The Facebook version of Candy Crush Saga currently has 440 levels, and a new set of levels is released approximately every 3 weeks. The smartphone versions of Candy Crush Saga currently have 395 levels.
The player is given a default maximum number of five lives. The player needs at least one life to play a level. If the player fails to complete a level, a life is deducted. Lives will regenerate at a rate of one life every 30 minutes. The player can also restore lives by asking Facebook friends for help, or by paying Facebook credits.
Special Candies are created when four or more candies of the same color are matched together. Special Candies have special effects (e.g. clearing an entire row of candies) when they are cleared. Examples of Special Candies include Striped Candies and Wrapped Candies.
If a player tries to swap two adjacent Special Candies, it will generally create a special combination effect. These effects are often more powerful than the individual candies' effects.
Obstacles appear in some of the levels. Obstacles have a wide variety of properties, and they are all designed to make it more difficult to complete a level. Examples of obstacles include chocolate that tries to slowly fill the entire game board, and tornadoes that randomly destroy spaces on the game board.
A booster is a single-use device that is costly (purchased) and can be used to help the player complete a level. Boosters are usually bought with Facebook credits. Every level in Candy Crush Saga can be completed without the use of boosters, but the game difficulty ramp is designed to encourage the purchase of boosters. Examples of boosters include a Lollipop Hammer that can destroy any game square, and a Free Switch that can swap any two adjacent candies.
In previous versions of Candy Crush Saga, a player could purchase charm items to help them complete a level. Charms were much more expensive than boosters, because charms could be used repeatedly. As of June 27, 2013, Charms are no longer purchasable.
Source: Wikipedia, "Candy Crush Saga", available under the CC-BY-SA License.