The Android Development document Tasks and Back Stack actually covers this pretty nicely .

The Android Development document Tasks and Back Stack actually covers this pretty nicely. A relevant excerpt: A task is a cohesive unit that can move to the "background" when users begin a new task or go to the Home screen, via the HOME key. While in the background, all the activities in the task are stopped, but the back stack for the task remains intact—the task has simply lost focus while another task takes place, as shown in figure 2. A task can then return to the "foreground" so users can pick up where they left off. Suppose, for example, that the current task (Task A) has three activities in its stack—two under the current activity. The user presses the HOME key, then starts a new application from the application launcher. When the Home screen appears, Task A goes into the background. When the new application starts, the system starts a task for that application (Task B) with its own stack of activities. After interacting with that application, the user returns Home again and selects the application that originally started Task A. Now, Task A comes to the foreground—all three activities in its stack are intact and the activity at the top of the stack resumes. At this point, the user can also switch back to Task B by going Home and selecting the application icon that started that task (or by touching and holding the HOME key to reveal recent tasks and selecting one). This is an example of multitasking on Android. Note: Multiple tasks can be held in the background at once. However, if the user is running many background tasks at the same time, the system might begin destroying background activities in order to recover memory, causing the activity states to be lost. See the following section about Activity state. Summary: The task is moved to the background when you press Home and sits in memory, not really doing anything and retaining its state. This means that (generally speaking) you can switch back to the task and it will pick up where it left off. However, the Android system can - and will - kill background tasks if it needs to reclaim memory. When and if this happens is entirely up to the system, and is probably why you see inconsistent behavior when resuming. If the task is destroyed by the system, it will have to be re-created when you launch it again. So unless the application author has taken measures to save the application's state during the destroy process, it will be lost (and saving exact state in things like games is fairly impractical). Another good (but also verbose) document is the one covering the Activity Lifecycle (there's a nice flowchart if you scroll down a bit).