Since April 24th for many website owner and web masters the World has com tumbling down. The cause is the Penguin Update that Google has implemented in order to cut down web spam. Earlier Panda Update was a major update many versions of which have been implemented.
From time to time Google unleashes these changes in order to improve the quality of sites on the top for the users. Most of these changes have come about through the algorithms but the Penguin Update is not one of them. It is more like a filter which checks for the web spam and qualifies or disqualifies the site, or on some instances lowered ranking may be the outcome.
Though the update has created lot of flutter especially from those who have been hit badly. But there have been lot of instances where good sites have been hit as well. Well this could be due to that one or the other element on their web page which may have been qualified as spam albeit it may have been in place unintentionally.
The finger clearly points out at duplicate content and too much formatting. Some on page factors like excessive use of H1 tag, bold tag, and yes keyword stuffing. Websites that do not pass through a good seo may contain lot of search terms, which most of the web designers add out of impulse or lack of knowledge. Many web designing companies do not have content writers, hence the site contents are usually palmed off from a similar site on the net. Improper contents arise due to bad quality and cheap content writers.
Hence the cleaning process would involve removing duplicate content, H1, H2 tags if too many and excessive internal links. Changing anchor text of internal links to natural and such that similar anchor text is not repeatedly used. There are many finer points to writing contents but that is for experts. You can write simple meaningful contents yourself which should be original and readable.
Ads: This is another issue that has come up in recent times. Your advertisements should be placed as such that they do not obstruct the contents or distract visitors in any way. The focus should not be selling advertisements or affiliate links but on creating a good website first.
After you have worked extensively on your website then wait for Penguin Refresh or the spiders to do their work and improve rankings.