Astroturfing is when the source of funding for a movement or an organization is concealed to make it seem like it has grassroots support.
A grassroots movement is one in which people, not money, are the basis for driving the direction of a movement. They are movements from the ground up, organic, natural, community-based, and participatory. A grassroots movement sets itself apart from other movements that are driven from the top-down.
Astroturf movements are driven from the top-down, but take great pains to create the appearance of a natural movement. Astroturfing is a facade, artificially contrived, and conceals its top-down agenda.