Searchable Subject Indexes
Searchable subject indexes make no attempt to index the content of a site. Instead, the name and URL of a site—and usually some kind of brief description—are included in a set of categories. The categories are usually browsable, but they're searchable as well. Yahoo and the Open Directory Project are examples of searchable subject indexes.
Mixing It Up
Now, here's the tricky part. Google, a full-text search engine, has a searchable subject component called Google Directory. Yahoo, a searchable subject index, has the option to search a full-text engine. (Yahoo's directory results are from their searchable subject index, while their full-text search matches are called Web results and come from their own full-text search ...