Just to confuse you with syntax, # can be comments in certain programing language, but not CSS. In CSS comments are wrapped with '/*' to start and '*/' to end.
/* COMMENT */
BLAH BLAH COMMENT
In CSS # indicates a id style while '.' indicates a class style. An id can only be used once in a page, while a class can be used repeatedly.
For example I could have a div (layer) of
<div id="blackbox"> </div>
then in my CSS sheet I would have to define that style. Suppose I wanted a black rectangle 20px by 30px. Then in my CSS sheet I would have the following:
Now suppose I want to keep reusing this element, I would want to make it a class instead.
In that case I would have the following in my CSS