please explain me how to find internal link and external link from our website .
"Internal links are links that go from one page on a domain to a different page on the same domain. They are commonly used in main navigation.
An external link is a link that points at an external domain." (Source: moz.com)
Google Crawler and spider are same phenomenon. Both find and fetch web pages.
Indexing is a process of arranging web pages in Google database on the basis of information found and fetched by Google Crawler or spider.