When you force a fluid to move with a fan or pump or some other contraption, you've got forced convection. When the fluid moves by itself due to temperature induced density gradients, you've got natural convection. When both are going on at the same time, you've got combined forced/natural convection.

Actually, every case of convection is a case of combined convection, since temperature gradients always exist. However, the relative importance of each can be different depending on various parameters. The dimensionless quantity to check to determine this relative importance is the Grashoff number divided by the square of the Reynolds number (Gr/Re²). If Gr/Re²  < 0.1 then forced convection dominates. If Gr/Re² > 10 then natural convection dominates. If 0.1 < Gr/Re² < 10, both are important and you are in the realm of combined convection.