Typography terminology

Typeface
A typeface is a set of characters sharing the same design. For example, Arial, Helvetica, Times and Verdana are typefaces.
There are two different categories of typefaces: serif and sans serif.

Font
A font is a specific size (usually defined in points) and style (roman, italic, bold, extrabold, condensed, etc.) of a typeface. For example, Verdana 12 pt bold is a font.

Serif
Serif typefaces use small decorative marks to embellish characters and make them easier to read. Times Roman is an example of a serif typeface.

Sans serif
Sans serif typefaces are composed of simple lines unlike Serif typefaces. Helvetica is an example of a sans serif typeface.

Times (serif type)	Helvetica (sans serif)
Roman	Roman
Bold	Bold
Italic	Italic

Baseline
The baseline is an imaginary line on which most of the characters "sit". The baseline is also relevant for measurements (including x-height and leading) and for alignment purposes.

Mean line
The mean line (or midline) is also an imaginary line running along the top of non-ascending, lowercase letters (e.g. "p" or "x") and at the curve of some ascending letters (e.g. "h").

Ascender
It is the part of a lower case letter (e.g. "t" or "b" which goes above the mean line.

Descender
It is the part of a lower case letter (e.g. "q" or "g" which goes below the baseline.

Point
Types are measured in points (pt). One point is equal to 1/72 of an inch (i.e. 0.3527 mm).

X height
It is the distance between the baseline and the mean line of the font. In most fonts it corresponds to the height of the lower case letter "x".

Font size
It is also called "point size" or "type size". It's the distance from the ascender line to the descender line. It corresponds to 1em.