Google and oher search engines need a way of finding a page/ site in the first place, eg. an external link from another site or submitting the site.

Google and others follows links according to "the rules" the website sets for robots. See http://www.robotstxt.org/wc/robots.html for more details and look at Mudcat's rules. This page for example has <META NAME="ROBOTS" CONTENT="INDEX,NOFOLLOW"> a "well behaved" (not all do as "instructed") robot would index this page if it found it but not follow any links contained in it.

Also, see Anahata's comments re frequency of indexing.