This is the Paragraph style located in the first drop down menu. This should be used for all your body copy. Links are displayed like this.
Obviously the lesser the number, the greater the importance. Most people don't go past the Heading 3 as the sizing of 1,2 and 3 tends to suffice. A good rule of thumb is to never use more than 3 fonts/font sizes on any one page anyway to help users read the content you've put up.