There are two major algorithm updates called PANDA and PENGUIN that refreshes the search index and deliver better and relevant results for the users query. Using these algorithm updates they find the websites who are against the Google webmasters guidelines and penalize those sites for being against to Google guidelines.
Panda: Panda attacks your website when your site have duplicate content. You will definitely get the ranking penalty by the panda for poor quality content and keyword stuffing.
Penguin: Penguin attacks when your site contains poor or bad back-links. If your site receives any unnatural link then penguin go bad and penalize your site, To overcome such term then you must carefully evaluate your link profile.
Bad backlinks:
* Paid Links
* Links from loss value directories
* Irrelevant links and the links from low-quality sites