It's funny how how some games rise upon other games' shoulders. I'm not saying that's a bad thing. It's just that these viral casual games, such as Candy Crush Saga, needed a successful predecessor in order to refine the model and become all the rage a few years later.
Candy Crush Saga is a colorful game about matching candy pieces in order to break them and score points. It was based on the fairly popular Bejeweled that came about a couple of years earlier. Here you go through a number of levels trying to accomplish specific tasks that become more varied as the game progresses. In each level you are presented with stacks of colored candy.
Your basic action is to swap one candy for another. In case this forms one or more lines of matching colors, the swap is successful and the candy pieces break. Lucky for you, there is enough candy to fill up the empty spaces. With this model, a game could theoretically go on forever, or rather until you end up with a matchless matrix. This scenario rarely happens in Candy Crush Saga however. The objectives of each level are usually completed way before your options become scarce.
There is not a lot a of depth in Candy Crush Saga. Actually there is none at all. There are simply too many colors and the view space is too narrow to be able to work a master combo. Combo's happen still. Mainly because you will (sometimes inadvertently) create special candy which has the ability to clear lines or areas of candy, all in the name of Tasty and points.
Candy Crush Saga has been out for more than 5 years. Developers have added a lot of extra content, and have created spin-offs such as Candy Crush Soda. Some find crushing candy incredibly fun. I for one would rather play Sudoku.