The difference between myosin and actin is that, where myosin uses cellular energy to move along the actin, the actin is the passive partner in the process. Myosin and actin are the protein types responsible for muscle movement, as well as movement in many other types of cells. Because they have such a widespread role, myosin and actin are crucial proteins for many forms of life.
The most evident example of the action of myosin and actin is in muscles. Myosin and actin filaments sit next to each other and slide past each other, shortening the muscle fiber they occupy. This is the basic mechanism of muscle contraction. Myosin has binding sites, known as heads, which bind the actin. When the myosin molecule is provided with ATP, the energy currency of cells, the head moves, pulling the actin filament along.
One of the most important functions of myosin and actin, outside of muscle movement, is in cell division. It is actually a ring of actin and myosin filaments, like a tiny muscle, which pinches two cells apart. This closes the cell membrane around each of the two new daughter cells, separating the cell contents completely. Thus, myosin and actin are vitally important, even to organisms without muscle tissue.