Active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration

Active transport is assisted by enzymes and energy. Since it requires energy, it is known as an active process. 
Since diffusion and osmosis do not require energy, they are known as passive processes. 

Examples of Active Transport:

1) Carbohydrates are broken into smaller sugars like glucose. Glucose is absorbed by active transport into the villi of the small intestine.

2) Plants take up mineral salts such as nitrates into their plant root cells by active transport from the soil. This is because concentration of nitrates is higher in the root cells and lower in the soil.