The (X)HTML specifications contain special codes for creating lists of items. You can create plain, numbered, or bulleted lists, as well as lists of definitions. You can also nest one kind of list inside another. In the sometimes sketchy shorthand of the Internet, lists come in very handy.
All lists are formed by a principal code to specify what sort of list you want to create (ol for ordered list, dl for definition list, etc.) and a secondary code to specify what sort of items you want to create (li for list item, dt for definition term, etc.).