W3C Specification: CSS 2.1 | Inline-level elements and inline boxes

Inline-level
elements are those elements of the source document that
do not form new blocks of content; the content is distributed in lines
(e.g., emphasized pieces of text
within a paragraph, inline images,
etc.). Several values of the ‘display’ property make an element
inline: ‘inline’, ‘inline-table’,
and ‘run-in’ (part of the time; see run-in boxes).
Inline-level elements generate inline
boxes.

Cascading Style Sheets, level 2 revision 1
CSS 2.1 Specification
W3C Working Draft 13 June 2005
Visual formatting model

http://www.w3.org/TR/CSS21/visuren.html#inline-level