A "grassroots" action or campaign is one that is started spontaneously, and is largely sustained, by private persons, as opposed to politicians, corporations, or public relations firms; a "grassroots" campaign comes about because of the popular feelings of some mass of people, as opposed to being the creature of the powerful.

"Astroturfing", then, is a campaign crafted by politicians or spin-doctors, but in such a way as to appear it's the result of popular feeling rather than crafty manipulation by political or corporate elites.

Astroturfing to one degree or another is increasingly common in American politics (and business). In reaction to the perception that politicians and corporations are not acting in the best interests of "the people", an astroturfing campaign attempts to gain legitimacy by appearing to spring forth spontaneously from "the people", like Athena from Zeus's forehead; when it's discovered that the campaign was meticulously crafted and manipulated by the same spin-meisters that the public has learned to distrust, the astroturfing adds to the very cynicism it was designed to circumvent.

One technique of an astroturfing campaign is to induce a number of its supporters to write email, letters to the editor, blog posts, crossposts and trackbacks, in support of the campaign's goals. The campaign instructs the supporters on what to say, how to say it, and where to send it, and above all, to make it appear that their indignation, appreciation, joy, or hate is entirely spontaneous and independent – and thus "real" – and not at all the product of an orchestrated campaign.

The idea is that the public at large will see lots of apparently "uncoordinated", "spontaneous" and "objective" viewpoints all in line with that of the astroturfing campaign, and will come to believe that if so many of their fellow "citizens next door" believe something, they should believe it too, for all those "independent" viewpoints couldn't have been faked.

With the advent of the Internet, it has become even easier to whip up an astroturfing campaign, as the cost of emailing – especially of emailing a pre-written, sign your name at the bottom email – is so low. The pervasive anonymity (or pseudonymity) of the Internet makes it even easier, as one person can play the role of a whole group of like-minded people. At the same time, the internet makes it easier for people to compare notes, making it somewhat easier to expose an astroturfing campaign.