What is osmosis and how does it work?

Osmosis is basically the movement of water across a semi-permeable barrier (i.e. a barrier that lets some molecules through but not others - like a cell membrane). Usually, the water muct move down something called a concentration gradient, which I will explain with an example.

Imagine that you have a tank of water, with a semi-permeable barrier splitting it in two. On one side of the tank, the water is 80% pure, but on the other side of the tank the water is only 40% pure. During osmosis, the water molecules will move through the semi-permeable from the area of high concentration (i.e. the 80% side) to the area of low concentration (i.e. the 40% side) until there is an equilibrium (i.e. until there is no overall movement of molecules). 

Hope that helps!