From the Wikipedia:
Astroturfing in American English is a neologism for formal public relations campaigns in politics and advertising which seek to create the impression of being spontaneous “grassroots” behavior, hence the reference to the artificial grass, AstroTurf.
The goal of such a campaign is to disguise the efforts of a political or commercial entity as an independent public reaction to some political entity–”a politician, political group, product, service or event. Astroturfers attempt to orchestrate the actions of apparently diverse and geographically distributed individuals, by both overt (“outreach”, “awareness”, etc.) and covert (disinformation) means. Astroturfing may be undertaken by an individual pushing a personal agenda or highly organized professional groups with financial backing from large corporations, non-profits, or activist organizations. Very often the efforts are conducted by political consultants who also specialize in opposition research.