I read in some answers about using the "Apple" + "Space bar" keys. Which is the "Apple" key?

Also, I see in my Mac OS X Safari menu bar that to open the download window I can use a 3 key combination. The last of the 3 keys are the Command key (depicted with a clover leaf symbol) and the 'L' key. The first key is the one I don't see anywhere. It is depicted by a symbol that looks to me like an upper case 'X' with most of the forward slash part removed. What key is that? OK, just discovered by trial and error that it must be a symbol for the "option" key. What is that symbol called and why is it not on the keyboard?