A serif font uses serifs, which in typesetting are small lines attached to the ends of letters or characters. You can easily notice the serifs when looking across the bottom of a group of words. The small lines work to connect the words together and draw a reader's eye across the page.
Examples of classic serif fonts include Times New Roman, Garamond, and Courier.
Note: howchoo does not use a serif font. Rather, we use a sans serif font.