Serif vs. sans serif
All typography can be divided into two types: serif and sans serif.
Serif typefaces have small finishing strokes, known as serifs, that extend from the ends of the letter strokes. Sans serif typefaces have no serifs and are less ornate than serif typefaces. Serifs help guide the eye horizontally, making serif typefaces more readable for long text blocks.
The bold statement made with sans serif type makes it more popular for headlines and display type.