Definitions

These use the tags dd, dl and dt - I'd never come across them before, but they are in my HTML pocket guide from 2002, so they're nothing new.

Foobar

Not a good state to be in

Windows

A pile of crap

Apple

Overrated

When viewed in Firefox 3 with no CSS, the above looks something like this:

Foobar
    Not a good state to be in
Windows
    A pile of crap
Apple
    Overrated

A CSS file I found for them turned it into 3 lines with the first item emboldened.

Confusingly there is also a <dfn> element which isn't related to definition lists, which seems to be intended to be used the first time a term is used. (Similar to how the first reference to something is a link in Wikipedia, but not subsequent mentions?)