If you have ICS you should have a virtual button or hardware key to display recent apps, and if you swipe them away it closes I think. The Android equivalent to the double-tap home button and touching and holding apps then hitting the red x on an iPhone.
Sometimes, in lieu of an exit option, hitting menu will bring a context menu, often with 'exit' being an option. Other times, hitting back will either silent exit or cause a 'hit back twice to exit' alert to display. Normally the back key only freezes apps so while running, they use no CPU or memory. I just hit back until the home screen displays if the app has no exit feature.
Samsung Galaxy S3 (Stock, rooted TouchWiz)
Samsung Galaxy Tab 3 7.0 (non-rooted, stock)
Samsung Galaxy Tab 2 10.1 (obsolete, slow, rooted)
Samsung Galaxy Note 10.1 2014 Edition