PageRank is Google's way of determining a website's worth based on the number of incoming links it has
. Essentially, Google counts the number of links pointing to the site and interprets it as confidence votes. Simply put, the more votes for a site, the worthier the site is in the eyes of Google.
During the years that the web was emerging, numerous sites that have industry-specific content were continuously being added to the web daily. Web surfers or searchers had very few tools to locate these sites which they knew existed but had no idea on how they can be accessed. The birth of Yahoo provided some relief as it organized its directory listing by classifying each site it discovered and likewise embedded a search engine in its site. This started the use of keywords existing in the database for site searching. Other search engines followed suit with the search trend and relied heavily on Meta tags to classify the relevance of a website based on keywords found in the tags.
Everything seemed to work out just fine until site owners and webmasters realized the potential of embedding industry specific keyword phrases in their Meta tags and other site codes to manipulate higher rankings in search results. Search engines started getting cluttered with sites that spammed their content with the abuse of relevant keywords. Most had the keywords but had poor content. The credibility and relevance of search engines were being challenged so they had to think of a way to offer a more refined output to users.
Google saw the problem which conventional search engines had to face in this situation. It recognized the fact that as long as the control of relevance remained with webmasters, the ranking results would continue to be contaminated with the presence of high ranking sites that artificially inflate their keyword relevance. By the very nature of the web, it is accepted that the web is based on hyperlinks where a site is largely measured by its linkage to prominent sites and the number of links it has. There is the assumption that a site is good and important if more sites link to it.