On your standard Linux or Windows desktop, you can have many applications running and visible at once in different windows. One of the windows has keyboard focus, but otherwise all the programs are equal. You can easily switch between them, but it’s your responsibility as the user to move the windows around so you can see what you’re doing and close programs you don’t need.
Android doesn’t work that way.
In Android, there’s one foreground application, which typically takes over the whole display except for the status line. When users turn on their phone or tablet, the first application they see is the Home application (see the figure).
When the user runs an application, Android starts it and brings it to the foreground. From that ...