GOOGLE PENGUIN
Google Penguin Update is a web spam algorithm update to penalized spam sites on the search results. As Google prefers on organic results, thus they always making updates to remove spam sites such as over-optimized, low quality or bad sites etc.
Why my site is hit by Google Penguin Update
If your site is hit by Google Penguin Update, then there must be a valid reason.
Your site may violating its quality guidelines.
Your site may showing Google different then your users which is called 'Cloaking' . Cloaking will definitely decrease your ranking on the search engines.
Your site may contain harmful elements such as Java Scripts, flash etc. that aren't crawlable by the boots.
Your site may involving in link exchange or receiving inbound links from bad sites.