A touch screen allows the player to use the mouse in order to interact with the game, usually by clicking on-screen buttons. The lower screen of the Nintendo DS is a touch screen.
Note that a touch screen is different to a dual screen. A dual screen simply involves dividing up the game screen into multiple parts. The Nintendo DS has both of these features, but they are by no means dependent on each other - you can easily have one without the other.
- You can always have a touch screen without having a dual screen.
- You don't need to restrict yourself to making just one part of the game screen touch-sensitive (i.e. having mouse-touch buttons in only one part of the screen). That was a restriction of the Nintendo DS, a restriction that Essentials games don't have.
- You can modify your touch screen to use double clicks and dragging movements. Think about how you could use these effects (e.g. moving Pokémon around in Pokémon storage).