What is the primary difference between a Web directory and a crawler-based search engine?

1. What is the primary difference between a Web directory and a crawler-based search engine?
2. What is the purpose of an index in a search engine?
3. Why are companies increasingly interested in enterprise search tools capable of handling unstructured data?
4. What is the difference between SEO and PPC advertising?
5. Describe three different real-time search tools