Firstly in your stylesheet the body does not need the #.
# is used to select an element as an ID. IDs can only be used once on any given page.
eg. <div id="main-content">BLAH BLAH</div>
Would be selected by
#main-content {} in your CSS.
For the body, you just use:
body {font-family:whatever;}
then if you want a different font in a particular div, you specify that div only in the div using the ID method above.
If you want to use similar styles repeatedly on a page, you would use a class. Classes are selected with a . in the css.
<div class="blue-text">This text is blue</div>
In your CSS would be:
.blue-text {color:blue;}