The first way of convection is forced convection. This occurs when a fluid is forced to flow over the surface by an external source. The motion which is created is an artificially induced convection current. An example of this type of convection is when a fan is blowing on a hot surface to cool it down faster.
The second way of convection is free convection, also referred to as natural convection. This is when the fluid movement is caused by the variations of temperature in the fluid. For example, when the fluid touches a warm surface, the molecules scatter which causes the fluid to be less dense. The colder fluid gets more dense and will sink. Therefore the more hot volume transfer heat towards the cooler volume of that fluid.