Search engines are used on the web to place the websites according to the ranking or suggestions.
Search engines can be placed on any site that needs to be made as search engine friendly. It allows the search engines to search the sites and display them on the front according to the pre-defined ranking criteria.
Once the site is suggested for the search engine then the website information gets collected and the links are followed on each web site.
The linked websites works when the site is visited and on that other links also gets active.
If there are more websites that link to the site of the user then the spider (information gatherer) will find the site and display it.