1. Desalination
Desalination is the process of converting saline (salt) water into freshwater.

2. Reverse Osmosis
Osmosis is water moving across a selectively permeable membrane to create an equilibrium.
In reverse osmosis, there is saline water on one side of the membrane and freshwater on the other. There is pressure applied to the salt water so nothing can flow there, and also it will flow to the side with freshwater so that just the salt is on one side and just freshwater on the other.
A diagram showing reverse osmosis.

3. Desalination by adding heat
When saltwater is boiled, the salt is left behind and then the water passes a valve. It is then condensed and ends up on top of the valve so it flows into a container.
A diagram showing desalination by adding heat.