Sans Serif Fonts
Sans serif typefaces have no serifs extending from the letterforms, and there's no thin/thick transition, which accounts for their monoweight appearance. This also means that sans serif typefaces look more alike than different. Compare the Arial on the left, below, with the Futura on the right.
Because there are fewer differences between sans serif typefaces, it isn't a good idea to use more than one per page.
In other words, you can combine an oldstyle and modern typeface because they have different structures, but sans serif typefaces have similar forms. If you put your headline in Arial and your subhead Futura, you'll create conflict, not contrast.
However, within each family of typeface, there are style variations that you can use to improve contrast. For instance, if you use Arial Bold for subheads, use Arial Black for headlines. A heavier weight will make the text appear thicker and darker, so that it stands out better. Differences in point size also create contrast.