If you've applied various formatting options to your text - eg: different fonts, font colours, font sizes or font styles (bold, italics, underline etc) - and then later on decide to apply some of these formats to other text, you don't have to laboriously go through the process of selecting the text, and then applying the various formatting options to it.  A quicker way is to use the 'Format Painter' tool. 

Step 1:
Place the cursor within the word you wish to use as the source of the format options.

Step 2
Click on the Format Painter tool.  Notice that the cursor changes into a paintbrush.  Word has read the format of the selected characters, and placed them 'into' the paintbrush.

Step 3
Drag the paintbrush over the text you wish to paint the format onto.

Step 4
Release the mouse, and the formatting changes take effect.

To repeatedly apply the formatting options to different blocks of text, DOUBLE-CLICK the Format Painter tool.  When you have completed painting this format onto new blocks of text, click the Format Painter tool once more to turn it off.