How the SEO work with crawlers

Crawlers (aka spiders) 'crawl' the web searching for pages. They of course won't find these pages unless someone else has linked to them or you've manually added your site to their search engine.

If one site gets a lot of links to it, then it's pagerank will (probably) improve.

However, this should not be your aim if you run a site. Your aim should be to get quality traffic so getting unrelated links is a waste of time. For example: a web site selling clothes shouldn't try getting links from sites about clocks (a bit random but thats the sort of thing some people try).

PageRank itself doesn't even accurately reflect search engine rankings as Google now uses hundreds of methods to determine search engine rankings, and pagerank is just one of them.

Your focus should be on giving your visitors good unique content, rather than on working on SEO tactics, some of which may cause you to get banned from Google (in extreme cases). If you provide content that other sites aren't, then people will link to you from their own sites because they think your site is useful, not because they want to increase pagerank

For more info on PageRank, try here and here