Below are some basic definitions to help you understand how type is described and measured.
TypefaceA typeface refers to a group of characters, such as letters, numbers, and punctuation, that share a common design or style. Times New Roman, Arial, Helvetica and Courier are all typefaces.
FontFonts refer to the means by which typefaces are displayed or presented. Helvetica in movable type is a font, as is a TrueType font file.
The different options available within a font make up a type family. Many fonts are at a minimum available in roman, bold and italic. Other families are much larger, such as Helvetica Neue, which is available in options such Condensed Bold, Condensed Black, UltraLight, UltraLight Italic, Light, Light Italic, Regular, etc.
Serif fonts are recognizable by the small lines at the ends of the various strokes of a character. As these lines make a typeface easier to read by guiding the eye from letter to letter and word to word, serif fonts are often used for large blocks of text, such as in a book. Times New Roman is an example of a common serif font.
Sans Serif Fonts
Serifs are small lines at the ends of character strokes. Sans serif, or without serif, refers to typefaces without these lines. Sans serif fonts are often used when a large typeface is necessary, such as in a magazine headline. Helvetica is a popular sans serif typeface. Sans serif fonts are also common for website text, as they can be easier to read on screen. Arial is a sans serif typeface that was designed specifically for on-screen use.
The point is used to measure the size of a font. One point is equal to 1/72 of an inch. When a character is referred to as 12pt, the full height of the text block (such as a block of movable type), and not just the character itself, is being described. Because of this, two typefaces at the same point size may appear as different sizes, based on the position of the character in the block and how much of the block the character fills.
PicaThe pica is generally used to measure lines of text. One pica is equal to 12 points, and six picas are equal to one inch.
The baseline is the invisible line on which characters sit. While the baseline may differ from typeface to typeface, it is consistent within a typeface. Rounded letters such as "e" will extend slightly below the baseline.
The x-height is the distance between the meanline and the baseline. It is referred to as the x-height because it is the height of a lowercase "x." This height can vary greatly between typefaces.
Tracking, Kerning and Letterspacing
The distance between characters is controlled by tracking, kerning and letterspacing. Tracking is adjusted to change the space between characters consistently across a block of text. This may be used to increase legibility for an entire magazine article. Kerning is the reduction of space between characters, and letterspacing is the addition of space between characters. These smaller, precise adjustments may be used to tweak a specific word, such as in a logo design, or a large headline of a story in a newspaper. All of the settings may be experimented with to create artistic text effects.
Leading refers to the distance between lines of text. This distance, measured in points, is measured from one baseline to the next. A block of text may be referred to as being 12pt with 6pts of extra leading, also known as 12/18. This means there is 12pt type on 18pts of total height (12 plus the 6pts of extra leading).
Gavin Ambrose, Paul Harris. "The Fundamentals of Typography." AVA Publishing SA. 2006.