Blood plasma is the yellow liquid part of blood. In normal blood, the blood cells are suspended in the plasma.
Plasma makes up about 55% of the total blood volume. It is mostly water (90% by volume) and contains dissolved proteins, glucose, clotting factors, mineral ions, hormones and carbon dioxide. Plasma is the transport medium for excretion.
In hospitals, blood plasma is extracted to either for medical therapy (plasmapheresis) or diagnostics tests (clinical chemistry, immunology,…) : an anti-coagulant stops the blood clotting. Then tubes of blood are spun very fast in a centrifuge. The blood cells go to the bottom of the tubes. The plasma is poured into another container.
Blood serum is blood plasma without fibrinogen or the other clotting factors (that is, whole blood minus both the cells and the clotting factors).