Active transport allows molecules to move from an area of low concentration to an area of high concentration, instead of the normal way down the concentration gradient. There are many ways of active transport but they all must use a transport protein. This is because of the polarity, size, ect. Proteins vary, however, there are protein carriers and protein pumps.
The sodium potassium pump is an example of active transport. In the sodium potassium pump, three sodium ions bind to the protein channel along with an ATP, which fuels the channel to change shape. This drives the ions through the channel and out of the cell. One phosphate group(from the ATP) remains attached to the protein. The change in shape allows the potassium ions to bind to the protein channel now. The phosphate group falls off and the two potassium ions are brought into the cell. In order for this activity to occur there must be a steady supply of ATP, because the pump is working against the concentration gradient.