What is the difference between plasma and serum?

Plasma is the fraction of blood that contains coagulation factors (clotting factors).
Serum is the fraction blood that contains all the ingredients as plasma except coagulation factors.

Clotting factors include factor XII, factor XI, factor X, factor VIII, factor VII, thrombin, fibrogen etc.

If fresh blood is added to a plain glass container, the top fraction obtained after centrifugation is serum. Coagulation factors are used to form a (fribinogen is converted to fibrin) fibrin clot which is trapped at the red cell fraction.

If frest blood is added to heparin, sodium citrate, or EDTA etc, the top fraction obtained after centrifugation is plasma. Coagulation (Clotting) pathway is blocked, the coagulation factors still remains at the plasma fraction.

SERUM IS PLASMA WITHOUT FIBRINOGEN.

It is because fibrinogen origenally exsist in plasma will become fibrin (insoluble) to clot , serum has no fibrinogen then.