Unlike the iPhone, the Droid can run more than one app at a time.
iPhone users making the transition, like John Gruber at Daring Fireball, sometimes say things like this: “I have no idea how to quit an app other than reboot.”
Don’t despair dear Droid newbie, Droidie is here to help.
Personally, I use Advanced Task Killer app to clean out the apps I don’t want running.
It’s pretty easy to use. Uncheck the apps you want to keep and click the button at the top of the screen and poof they go to heaven until you run them again.
If you want to quickly see which apps are running, hold down the Home key at the bottom of the Droid.