Gravity - a constant pull downward (along the negative Y axis).
Uniform - a push along a certain direction.
Fan - a push outward in a cone shape from a point, with a falloff based on distance.
Drag - a retardant force that slows the forward motion of an object. This is not the same as air resistance, which would take into account the profile an object presents to the air.
Fluid force - pushes objects based on the motion of a fluid simulation.
Vortex - a tornado-like swirling force.