Macintosh computers have a special command key marked by a four-leaf clover or an apple. The Command key is similar to a PC's Alt key -- you hold it down while pressing another key to execute some operation. Typically, command-key combinations are shorthands for menu choices. For example, on the desktop, pressing the Command key and O is equivalent to selecting the openoption from the file menu.
The Command key is sometimes called the Apple key or Open Apple.