A standard keyboard can logically only display so many character inputs.
To work around this limitation, Windows has a shorthand method for accessing hundreds of other characters using the Alt key and the numeric input pad on the far right.
To input the Alt Character code of your choosing, hold down the Alt key while inputting the corresponding reference code.
(This only works with the number pad so make sure to have Num Lock on).
A list of the four digit numeric reference codes can be found here.
On Mac OSX the method differs slightly.
In order to access these additional characters you’ll need to locate and hold down the Alt/Option Key and combine with another letter or number key.
You can also add the Shift key to access another set of characters.
Here you can find a brief explanation and outline of the Mac OSX Alt characters.