Serif and sans serif are two common styles of letters. A serif letter has small finishing strokes at the ends; a sans serif letter does not. It is a bit like comparing a carved sign with a clean painted sign.
Many books use serif letters, while many screens use sans serif letters, but either style can work in both places. The choice affects the feeling and readability of a design.
Pick a type style that is easy to read at the size people will see it. A decorative style should not make important instructions difficult to understand.