Two types of convection are forced convection and natural convection. Forced convection occurs when flow is caused by an external device such as a fan. Natural convection is when flow is caused by thermal gradients in a fluid.
Many systems have both natural and forced convection.
There are two boundary layers associated with convective heat transfer. One is the thermal boundary layer and the other is a velocity boundary layer. The thermal boundary layer describes how the temperature changes from the contact surface out into the fluid. At the contact surface, the temperature of the fluid is the same as the temperature of the surface. The velocity boundary layer describes the change in velocity outward from the contact surface.