Difference between Crawling and Indexing?
Crawling means if Google bot (softwares) visits a page in your site and reads the content inside it is called crawling.
Indexing refers to when Google saves your site address in its index (collection of webpages, content, anchor text).
What is NoIndex Meta-Tags:
The ‘Noindex’ allows search engines to read the pages but instructs them to remove it from memory that it was ever indexed. That means when the search engine comes to a page with noindex meta tag, it will continue to read the content inside it including the links (so link juice is passed) but will forget it after reading and will not index it.
What is NoFollow?
If a search engine finds a “no follow” backlink it will just ignore the link and will not get deeper into crawling and no link juice is passed.
What is DoFollow?
When Google bots are crawling a website they look for external links point to other websites. If this links is “do follow” then search engines will follows the link and so link juice gets passed.
How different search engines treat Nofollow?
Google: Google does not follow the nofollow link.
Yahoo: Yahoo follows that link for finding new content but link juice is not passed.
Ask: It does not follow nofollow tag and treats it as normal dofollow link.