1. The use of paid shills to create the impression of a popular movement, through means like letters to newspapers from soi-disant ‘concerned citizens’, paid opinion pieces, and the formation of grass-roots lobbying groups that are actually funded by a PR group (AstroTurf is fake grass; hence the term). See also sock puppet, tentacle.
2. What an individual posting to a public forum under an assumed name is said to be doing.
This term became common among hackers after it came to light in early 1998 that Microsoft had attempted to use such tactics to forestall the U.S. Department of Justice's antitrust action against the company. The maneuver backfired horribly, angering a number of state attorneys-general enough to induce them to go public with plans to join the Federal suit. It also set anybody defending Microsoft on the net for the accusation “You're just astroturfing!”.