Active transport refers to a process in which a substances is transported across a membrane by the use of an integral membrane protein (or protein complex) that has ATPase activity.
The ATPase activity of the transport protein makes it possible for the substance to be transported against a concentration gradient of that substance. ATP is required because the movement of a substance against a concentration gradient of that substance is an endergonic process.
There are several active transport proteins in cells.
Na+ - K+ ATPase
H+ ATPase (proton pump)
H+ - K+ ATPase
Na+ - K+ ATPase animation