Hi allo, as has been said, the middle (home) button is the equivalent of the windows 'x' button, for a more permanent close at the end of use, which also helps battery life, double click the home button quickly, then when the icons show press an icon and hold till they wiggle, a red 'x' is displayed, press it on each icon and this will close them in the iPads memory, it helps to speed up the iPads processing too.
Remember to save any unsaved info in the programs before you permanently close them.......don't want to lose valuable info!