Why you should use sans serif fonts for figures, posters, and slides.
Serifs are those little vertical lines and flourishes at the ends of letters (like the vertical lines at the ends of the capital S or the horizontal line at the bottom of the lower-case r).
Use sans serif fonts (Helvetica, Arial) because the near-uniform width of the strokes keeps the font readable when reduced in resolution or reduced in font size. Sans serif fonts are more legible from farther away, which is why they are good for posters and slides, particularly the titles and headers.
Serif fronts (Times, Optima) are better for reading large blocks of text where the serifs help to keep your eye focused on a line of text, helping you to recognize the word as a shape, which enhances your ability to identify the word and read quickly.