Diffusion is the movement of particles from an area of high concentration to areas low concentration.
For example, if you are baking brownies and your brother is in the next room, you will smell the brownies before he does. However after a while, the aroma will get to him and he will come running into the kitchen for a taste.
Another example: If you place a drop of red food colouring in a glass of water and leave it undisturbed, eventually the entire glass of water will have a red tint. The food colouring moves through the water until it is equally distributed throughout the glass.
Diffusion takes place along a concentration gradient. A concentration gradient exists until the diffused substance is evenly distributed. The movement of these molecules is said to be passive. No energy is required. The natural movement of the particles supplies the energy.
Some examples in science are:
1. Carbon dioxide entering the stomata of leaves.
2. Oxygen entering the bloodstream (and carbon dioxide leaving).
Visit How diffusion works and play through the animation.