Static loads are loads that exert a constant amount of force, while dynamic loads exert varying amounts of force upon the structure that is upholding them. A simple example of a static load is the weight of a bridge span upon the bridge pilings holding it aloft. A truck moving across that same bridge span would be exerting a dynamic load on the pilings.
Other examples of dynamic loads include wind blowing on the face of a building, water sloshing in a jar and dice bouncing up and out of a can. The variations of dynamic loads can be due to changes in mass, direction of movement, pressure or speed of movement.