Google Crawler and or spiders or human coded machine running under linux who visit websites according to there importance and ranking . Crawler scan a website by images , text , links , and video etc . They then cache website into there system and show users when they search by keywords and or images or video search for example google images or google video etc . Crawlers rate your website by detecting the number of back links to your website and quality of those back links . For example if your website have a links back from a PR 7 website then google give u a big boost and importance will be high . Crawler also rank website by there type like .edu .gov. .org .com .net etc . .gov and .edu website are on top priorities . Hence crawlers are spiders who time by time visit website and search for new content and index them for searchers .
You can rent or purchase high PR one way links from link sellers . OR you can search high PR blogs & links pages and then post your link either by commenting or requesting for it to webmaster of that website . You need couple of searching skills on google to achieve your target .