Google’s algorithm is the program used by Google for ranking websites on its result pages (SERP) among organic results. Theoretically, the first page is more relevant for the query.
Several hundreds of factors are used by Google’s algorithm to assign a rank to a page for a given query.
It seems Google’s program is more of a set of algorithms and filters than a real unique algorithm.
The first step is to calculate a relevancy score for pages according to the query. It is done by analyzing page content and popularity.
Popularity is based upon volume, structure and quality of backlinks and is determined by the well known PageRank algorithm.
Different filter algorithms are applied to candidate webpages. Those filters analyze page layout, loading speed, quality content and eventually penalyze some pages. The most famous filter algorithms are Google Panda and Google Penguin.
Google’s algorithm treatments are different according to the type of search:
vertical search (image, video, news,etc.)
Google’s algorithm is virtually changing every day and is scrutinised carefully by SEO professionals.