What is osmosis?

Osmosis is a special case of diffusion, which you can imagine as a spreading of particles across a room for example when you spray some deodorant - the strongest smell is closest to the area that has just been sprayed as there are lots of particles here but as the particles spread across the room, the strength of the smell reduces as there are fewer particles close together.
In osmosis, we are talking about the diffusion of water molecules, which move across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration. This results in a more equal spread of water molecules. Water concentration is the amount of water compared with the other molecules around, such as salt or glucose, known as solutes, which become diluted down by the water.