What is the difference between primary and secondary active transport?

Primary active transport directly uses energy (usually from ATP hydrolysis) to move molecules against their concentration gradient (e.g., sodium-potassium pump). Secondary active transport (or co-transport) uses the energy stored in an electrochemical gradient (often created by primary active transport) to move another molecule against its concentration gradient. It does not directly use ATP but relies on the gradient established by primary active transport (e.g., Na+-glucose co-transporter).