Beginning in February 2011, Google began implementing a series of changes to its search algorithm that remove more low-quality sites from search results. The update, termed “Google Panda,” correspondingly rewards high-quality websites.
Google Panda 4.2 is being phased in slowly over the coming months. If you see the search ranking for your website change abruptly, the update is the likely explanation. The updated algorithm penalizes spelling and grammatical errors, as well as slow speed and redundant content. Search Engine Land provides a comprehensive guide to Panda. It will answer many of your questions.