HTML uses tags to describe content.
<h1>This is my headline </h1>
<h2>This is a sub-heading </h2>
<p> This is my paragraph</p>
CSS is used to describe content for the tags.
An easier way to read this code is to write it like this:
You can also create a "class" of text. Below I created a class called "code".
In the middle of this sentence, the colour will be different
<p>In the middle of this <span class="code">sentence, the</span> colour will be red</p>
This was written in my CSS document like this:
Try to keep your CSS in one document, and attach it to your web page. Text > CSS Styles > Attach Style Sheet...
For more detail on CSS, go to http://www.w3schools.com