In CSS classes are used when you want some text to be different from another. For example say I was making a table and I wanted one cell to have a different bg colour than my other I would make a class.
To make a class put a fullstop(.) and then the name of the class then a { and then put what you want in the class.
example
.cell{ background-color:pink;
border:2px dotted hotpink;}
remember to end the class with a } and separate each line with a ;
To make the class show up on your page you have to put class=CLASSNAME, like below <table><tr><td class=cell>TEXT</td></tr></table>
But if you class isn't for a table you can write it like this;
<p class=cell>TEXT</p> or <div class=cell>TEXT</div> NOTE: You don't have to use cell as the name of your class cell is just an example