[RKDinOKC]

Serif fonts were designed for books to help keep your eye focused on the line you are reading.
San-serif fonts were originally to stick out as headlines and text with only a few paragraphs (news papers)
Official letters were typically in Serif as that is what type writers used.
Now with email more prevalent San Serif are much more the norm for official letters and are typically the default in email programs.

The key is to NOT use too many different fonts on a single page, then it becomes confusing and messy looking despite what some artists say (lol).

The exception being decorative fonts used for headlines to separate articles (newletters and magazines).

The most common sans serif fonts are Helvetica and Arial.
The most common serif fonts are Times and Palatino.
Those are available with many different varieties of lightness, boldness and condensed or expanded.

Others are used, but are mostly a personal preference. Typically a company sets a default font for it's body text and a couple for headlines, etc. They are part of the companies logo/letterhead image package and are to re-inforce a branding look and feel.

When I was supervisor of a companies drafting department all published drawings and illustrations were inked with leroy lettering templates. While supervisor we went from penciled mechanical drawings and inked illustrations to Computer Aided Drawing and Design, CADD. I personally using a program called Fontographer created a couple of variations of Leroy Lettering as fonts to be used by the CADD systems. Had originally found a company that I sent all the characters in leroy lettering and they sent me back a font. I say I created the fonts because I had to fix most of the characters and set the font spacing to work correctly (the difficult part). Then I developed bold, italics, and condensed versions. Did it basically to help tie the manual drawings and computer drawings together. The company still uses those fonts for drawings and illustrations today.