The Character Palette is a feature of Macintosh OS 10.2 and higher. It is a window that floats above all other windows on your screen and allows you to double-click any character to insert it to your document. The palette can be arranged by character type or by Unicode charts, depending how you would like to find it. It's handy for those characters that you don't need to use very often and do not know how to input through the keyboard.
The Character Palette in OS 10.3.x has the additional feature of showing you an example of each character in the fonts that it is available in. This makes it incredibly easy to find and input rarely used characters.
To make the Character Palette accessible, go to your System Preferences > International > Input Menu and check the box next to Character Palette. This will add it to your keyboard menu at the top of your screen.