How does osmosis differ from diffusion?
The difference is some kind of wall between the two substances.
If we put a layer of (clean) water on top of a layer os salt water, the salt will be evenly distributed throughout, af a while. This is called diffusion.
If we separate the salt and normal water by a semi-permeable wall that will only let through the water molecules and not the salt ions, water molecules will tend to migrate to the higher concentration in order to dilute the salt water. If the wall were vertical the liquid in the salt water compartment would be higher, but only to a certain level. This is caused by osmotic pressure.