According to wikipedia
PageRank is a link analysis algorithm which assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set. The algorithm may be applied to any collection of entities with reciprocal quotations and references. The numerical weight that it assigns to any given element E is also called the PageRank of E and denoted by PR(E).
In short it is based upon mathematically formula which use uses links as votes (more votes mean you are at top of the Google search engine).
This article explains
=> The myth
=> The algorithm
=> And the actual 126 line python code for Pagerank.
A great read to understand both math and Python code behind Pagerank. More information about PageRank can be found at following location:
=> PageRank from Wikipedia, the free encyclopedia
=> How Google Finds Your Needle in the Web's Haystack