Typically, icons look like this:
To make a program window into an icon (i.e. to minimize, or iconify it), you can either use the Minimize function on the window's Menu Button, or simply use the minimize button at the top right corner of the window.
Your program can keep working while it is iconified, so if you ask Maple to factor some huge number which takes half an hour, you don't have to leave your screen cluttered up for fear Maple will "freeze" if you iconify it.
To deiconify, or maximize the program, just double-click the left mouse button over the icon. By default, icons tend to congregate near the lower left corner of the screen.
Single-clicking on an icon accesses the menu button menu.