Search Engines have spiders which index websites over the internet. If your site links to a new site, then the new site will get indexed or spidered/crawled by the search engine bots or spiders when they visit your site. This is why it is essential to get links to your new site in order to get it indexed on the search engines.
A "search engine" is a Web site that employs bots to search the Web. Search engines take the information gathered by its bots and use it to create a searchable index of the Net. The "search" in search engine refers to the searching the bots do, not the searching you do to find things on the (search engines are often organized solely by keyword search)....
There are two ways a search engine can find a website to add to its database. The first and most direct method is to accept submissions from website designers and optimizers. Most search engines have a "submit your URL" section inviting submissions from Internet users. The second, and least obvious method of finding websites is to send a "spider" to follow each and every link on, off, and through a website. New URLs encountered by the spider will be added to the database. In this way, some search engines will grow very rapidly.
is it possible if you have zero back links for a site, it will never be crawled by Google Spider or by any search engine