An Cascading Style Sheet (CSS) is basically styles of a page within a block of text. The cascading part means that the last value for a set style will be the value taken, e.g. if at the top of a style sheet there is a value of black for the back ground colour and then white is below that, the white value will take precedence over the black.
Here is a example of styling within the tag body.
And this is an example with CSS within the html code.
<title>Background colour with css</title>
background : #DCDCDC;
There are many styles options within the html styling list, hopefully shall cover most of these in the future.