Last week Google has announced changes to its search engine algorithm. According to many internet sources, the change it is going to affect more sites in a shorter time than normal.
Google wants provide original content of interest to readers and spamming pages would put up pages with content scraped (stolen) from other sites and present it as its own.
The change is designed to remove low-quality content from its top search results. That means that content farms may be the primary target here.
To figure out who was hit and who gained it’s worthwhile to read a great recent article by Danny Sullivan at Search Engine Land