What do web crawlers do?

They install themselves on your computer to deliver advertising. They are adware.
They index web pages for the search engines. They are a kind of database software.
They follow links on the web and send back information on web page contents to search engines.
They are software that search engines use to find and deliver the results of queries.

The correct answer is:

They follow links on the web and send back information on web page contents to search engines.