Conducting a Productive Web Search
http://owl.english.purdue.edu/handouts/research/r_websearch.html

Purdue University
1998

Abstract:

There are two main types of indexes: those that are hierarchical (i.e. that lead one from a general topic to a more specific one) and those that list sources in some sort of order (most commonly alphabetical).  The first type of index often contains a broad range of topics while the second are usually sources designed to address a particular topic or concern.