What is the difference between active transport and diffusion.

Diffusion is the passive (does not require energy input) movement of a substance down a concentration gradient from an area of high concentration to an area of low concentration. Eg how the colour and flavour molecules spread throughout a whole glass when you add water to squash.
Active transport is the active (requires energy input) movement of a substance against a concentration gradient from an area of low concentration to an area of high concentration. It requires specialised cell surface proteins or carrier molecules to achieve this. Eg movement of ions from a low concentration in the soil to a high concentration in a plants root tip.