What's the difference between primary and secondary active transport ?

-In primary active transport, the energy released by ATP hydrolysis drives solute (S1) movement against an electrochemical gradient. - In secondary active transport, a gradient of ion X (S1) (often Na+) has been established by primary active transport. Movement of X (S1) down its electrochemical gradient now provides the energy to drive cotransport of a second solute (S2) against its electrochemical gradient.