Plowing is becoming a lost art in my opinion. Since a picture is worth a thousand words, take a look at Scott C's post about plowing from a while back.
A brief description, by no means complete, would be that the point of the plow enters the ground when the plow is lowered. The plowshare slices off a section of earth and the moldboard turns it over. The landslide runs along the wall of keeping the plow in line. If the point of the plow is too low, the plow will dig in. If the point is too high, the plow will not stay in the ground. This is controlled by the depth control lever or plow adjustments.