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.
Background colour
hi world
And this is an example with CSS within the html code.
Background colour with css
Hi World
There are many styles options within the html styling list, hopefully shall cover most of these in the future.