Pet Rescue Saga is a puzzle game that takes place in a fictitious cartoon world. Each stage (similar Candy Crush) asks you to complete a specific task, except instead of eliminating jelly from the screen or moving ingredients, you destroy multi-colored blocks to help 10 cute and cuddly animals reach an exit, among other objectives.
You do this by simply grouping blocks of the same color and then clicking to make them disappear. Without the help of boosters (the pickaxe, for instance), you can only blow up groups of two or more, so your primary goal will usually involve gathering as many blue, purple and green boxes together to achieve the highest score.
Like Candy Crush, you're free to play Pet Rescue Saga without spending a dime, though King intentionally baked different boosters into the experience that you can only acquire via in-app purchase.