Diffusion is a mixing process that occurs in liquids or gases put em together and over time they will mix themselves until equilibrium is reached.
Take a glass of water, put in a drop of food color. Wait. Diffusion mixing is a product of the vibration of molecules, all molecules vibrate if they are hotter than 0 degrees K. Think billions and billions tiny little paint shaker machines.
Moisture vapor is gaseous. Just like the food color, it will move from high concentration to low until eventually the concentration is the same.
Capillary action is the movement of liquid water. It is a product of surface tension. Think of a chain pulling all its little links along with it.