. = class
You can use classes as often as you need and in different ways

[blank] = just a regular declaration

--

#box {-details-}
<div id="box">
You can only have one such box used once on a page.
</div>

.text2 {-details-}
<div class="text2">This could be used here</div>
then we have some other text
<div class="text2">and we use that styling again here</div>

h3 {font-size: 10px;}

.bigger {font-size: 20px;}

--
You can mix though ..

<h3>This will be 10px tall</h3>
<h3 class="bigger">and this would be 20px</h3>

or you could have

.red {color:#ff0000;}
.green {color:#00ff00;}
.blue {color:#0000ff;}
and then do
<p class="red"> Have some red <p class="blue"> and some blue etc...

Silly question about CSS. I'm looking at a CSS file and am wondering when you use the # marks, when you use the periods and when you don't use anything at all in front of the lines.

Thanks,
-drmike

Told you I didn't know anything about CSS :)