"Convection" refers to vertical movement within a fluid (or a plastic-enough solid), caused principally by gravity.
- Some fluid moves upward because it is lighter than the surrounding fluid.
- Some fluid moves downward because it is heavier than the surrounding fluid.
Many fluids are lighter at higher temperatures and heavier at lower temperatures, and most convection that people experience every day results from density changes due to differential heating.
Every convection column has a "top" and a "bottom". This can be a source of heat, or a solid obstacle such as the surface of the Earth. However, such an obstacle is not necessary: At some point, the moving fluid will transfer enough heat to the surrounding fluid that its impulse to move will go away.
Moving fluid causes pressure differentials that either push other fluid out of the way, or suck more in from behind (Venturi flow). In the middle of a column, this comes from above or below, but at the bottom or top of a convection column, this has to come from the sides. So, convection triggers horizontal motion, known as advection. Adjacent upward- and downward- moving convection columns are eventually connected by advection currents into loops of moving fluid caled "convection cells".
<- <- <- <- <. .> -> -> -> -> -> -> -> ->. .<- <- <- <- <-
<- <- <- <- ^ -> -> -> -> -> -> -> -> v | v <- <- <- <- <-
^ | ^ Advection (Cooling) v
C | | | | C
o ^ v | v o
n ^ | ^ v n
v | | | | v
e ^ Cell v | v e
c ^ | ^ v c
t | | | | t
i ^ v | v i
o ^ | ^ v o
n | | | | n
^ Advection v | v
> -> -> -> ->^ | ^<- <- <- <- <- <- <- <- < v > -> -> -> -> ->
-> -> -> -> -' `- <- <- <- <- <- <- <- <- -> -> -> -> ->
Heat
Source
Although gravity is a requirement, other forces can cause interesting analagous effects, such as the electrical currents induced in a metal disk rotating through a magnetic field.