What is the difference between search engines and directories?

The main difference between a search engine and a general directory is that a directory will not list your URL if you do not first register it with them. They do not make use of indexing software agents and so have no way of knowing it’s out there. As a result, their registration form will be considerably longer than just your URL. Directories are usually subdivided into categories and you have to submit your URL under the most appropriate heading.