0. The purpose of styling is to make something stand apart (which can also be achieved with just proper spacing) - nothing will stand apart if too many different styles are applied to different pieces of text.

1. Avoid using many fonts on the same page

2. Avoid all capitals and too many capitalized words

3. Use small capitals for capitalized words

4. Avoid underlining; use italics or bold, or a different color, instead

5. Avoid mixing styles (use bold only instead of bold and italics together)

6. Fonts convey certain characters; use the right one. For example, the Comic Sans font is inappropriate for a formal presentation.

7. Left-aligned text is easier to read than a right-aligned one or a justified one

8. Serif fonts are usually easier to read than sans serif fonts at small size (regular text)

9. Serif fonts are usually as easy to read as sans serif fonts at large size (headlines, titles, etc.)

10. Avoid using similar fonts together on the same page

11. Use lines, bullets (like •), reversed text (like this) and indents for styling

12. Use em- (—) and en-dashes (–) instead of hyphens (-) for the minus (-) (as it usually appears too short and may even be inclined in some fonts) and follow (as in 'see below -') signs.

A font or a typeface is a set of graphic symbols, each representing the alphabet (a, b, c...; lower and upper cases), numbers (1, 2, 3...), symbols such as the dollar sign ($), etc. All symbols in a font share a unifying theme. It is thus possible to identify the font from just one or a few of its characters. Below are nine characters respectively in the Helvetica, Times and Comic Sans MS fonts

Fonts are commonly classified into serif and sans serif. Serifs are the little cross extensions at the end of a character's stroke. Serif fonts include Times, Georgia and Times New Roman. Sans serif fonts include Arial, Helvetica, Lucida Grande, Monaco and Verdana. Below are some characters in Times or Arial; one of serifs, or its lack, is pointed out.

Slab serif fonts have serifs that are not angular. An example is the Litho Antique. Script fonts form their own category - they are fonts such as Brush Script and Zapfino that try to emulate handwriting.

Fonts are also classified into mono-spaced (fixed width) and proportional types - in the former, all characters take the same width. Monospaced fonts such as Courier and Monaco are therefore used when aligning sequences. Compare Courier with Geneva below

Also see this Wikipedia article and this pimer from Adobe (PDF).

Note some terms such as cap-height and ascender used to define characters of a font below

Also note terms such as ligature, kerning, leading and line-spacing below