In iOS6 double tapping the home button showed all the apps running in the background. Holding down your finger on one of them showed little "X"s on the top left of each icon, so you could kill it. I don't see how to do that anymore. Any ideas?
iMac 3.06 GHz Intel Core i3, Mac OS X (10.6.8), 12GB RAM, iPhone 3gs, iPhone 4s