A simplified explanation of the Android lifecycle
If you have ever used an Android device, you have probably noticed it works quite differently from many other operating systems. For example, you could be using an app – say you're checking what people are doing on Facebook. Then, you get an email notification and you tap the notification to read it. Midway through reading the email, you might get a Twitter notification and because you're waiting on important news from someone you follow, you interrupt your email reading and change apps to Twitter with a touch.
After reading the tweet, you fancy a game of Angry Birds but midway through the first fling, you suddenly remember that Facebook post. So, you quit Angry Birds and tap the Facebook icon.
Then you resume Facebook, probably at the exact same point you left it. You could have resumed reading the email, decided to reply to the tweet, or started an entirely new app.
All this backwards and forwards takes...