As defined by Merriam-Webster.com, astroturfing is "organized activity that is intended to create a false impression of a widespread, spontaneously arising, grassroots movement in support of or in opposition to something (such as a political policy) but that is in reality initiated and controlled by a concealed group or organization (such as a corporation)."
Astroturfing can be used in a variety of ways. This is a sampling of news articles about different types of astroturfing campaigns.
Academic research on astroturfing has also been conducted. Again, this is just a sample of what is out there.