Osmosis is the one-way movement of gases or solutions through a membrane. The reason for this movement of the atoms is diffusion.

Osmosis power plant (by Wicks and Loeb):

If there is water with different saline content the water has a tendency to mix until both water components have the same saline content. In the upper picture the fresh water tries to mix with the salt water. Only the fresh water particles can pass the membrane and can get into the pressure chamber. The salt water particles cannot pass the membrane. Because the fresh water particles pass into the pressure chamber the pressure rises. The pressure can drive turbines in order to generate electricity.

These power plants can be used in coastal regions where rivers flow into the oceans.