A protein that makes up one of the three major structural elements in eukaryotic cells. Besides the actin cytoskeleton, there are intermediate filaments and microtubules.
The actin cytoskeleton appears under the microscope to be located just under the plasma membrane; that is, it forms a sort of second membrane. In adherent cells, they also form stress fibers along the surface that is attached.
Actin is implicated in chemotaxis, phagocytosis, pinocytosis, membrane ruffling, and pretty much anything that requires the shape of a cell to change.
Interestingly, the toxin phalloidin is used in conjunction with fluorescent dyes to visualize the actin in cells in microscopy. The toxin comes from the mushroom Amanita phalloides, and it kills you buy mucking with your actin cytoskeleton. Kids, do not eat this at home.
Another very intersting fact is that the pathogenic bacterium Listeria monocytogenes utilizes the host actin in order to move around. It makes a protein at one end of the bacterium named ActA which induces actin polymerization. This gives a ratcheting action to its random motion within the host cell (caused by the host cell itself, or perhaps Brownian Motion.