These come in many variations but the core principles are the same. Enemy mobs follow a predefined path on a map. Whenever they reach their destination, the player will lose something (often a life). You as the player want to avoid these enemies from reaching that point. The player will build a variation of towers, that will shoot these enemy mobs, to kill/destroy them before they reach the end of the path.
On top of this, a lot of things can be added to make the playing experience more unique. These can be things like
As you might have expected by the name of the game, you will be shooting at ships in All Ships Must Sink. These ships will sail alongside your beaches, on which you can place your towers. Destroy the ships before they reach the end, sounds easy right?