To sum up what a Style Sheet would do, consider the following:
The Red, Shiney, Fresh, Ripe, Big, Clean, Juicy, Tangy, Bright Tomato was in the supermarket. Someone picked up the The Red, Shiney, Fresh, Ripe, Big, Clean, Juicy, Tangy, Bright Tomato. They got home and ate the The Red, Shiney, Fresh, Ripe, Big, Clean, Juicy, Tangy, Bright Tomato.
Now that seams a bit long winded doesn't it?
With CSS it would be more like this:
Tomato = Red, Shiney, Fresh, Ripe, Big, Clean, Juicy, Tangy, Bright
The Tomato was in the supermarket. Someone picked up the Tomato. They got home and ate the Tomato.
CSS generally makes smaller pages, thus making the output quicker. CSS can also be used instead of tables to position elements, however I recommend learning the basics first otherwise it like learning to dive before swimming.
If you need some help, feel free to PM me. I recommend the link:
http://www.w3schools.com/css
As for your previous question, you can just use CSS in the same document...
<style type="text/css">
Blah
</style>
Just place that in between the <head> tags.
Alternativly you may use the previous method, with the path or URL to the style sheet.