Blood plasma, or plasma, is prepared by obtaining a sample of blood and removing the blood cells. The red blood cells and white blood cells are removed by spinning with a centrifuge. Chemicals are added to prevent the blood's natural tendency to clot. If these chemicals include sodium, than a false measurement of plasma sodium content will result. Serum is prepared by obtaining a blood sample, allowing formation of the blood clot, and removing the clot using a centrifuge. Both plasma and serum are light yellow in color.
Plasma is the liquid portion of the blood that is separated from the blood cells by centrifugation. One of the characteristics of plasma is that it clots easily which is important for hemophiliacs needing a transfusion but is a nuisance in most other applications. By agitating the plasma, one can precipitate the clotting factors as a large clot, and the leftover fluid is called serum. So, serum plus clotting factors is plasma, and clotted plasma yields serum (as an interesting aside, "serum" is Latin for whey, the liquid portion of clotted milk removed in making cheese).
The following course note describes the contents of the blood, plasma, and serum.