Imagine surfing the Web, going from page to page by randomly choosing an
outgoing link from one page to get to the next. This can lead to dead ends at
pages with no outgoing links, or cycles around cliques of interconnected pages.
So, a certain fraction of the time, simply choose a random page from the Web.
This theoretical random walk is known as a Markov chain or Markov process. The
limiting probability that an infinitely dedicated random surfer visits any particular
page is its PageRank. A page has high rank if other pages with high rank link to
it.