Blood plasma and blood serum are both devoid of blood cells and platelets.  The difference is in the clotting factors. Blood plasma still contains the proteins that allow it to clot: fibrinogen and other clotting factors, whereas blood serum has had the clotting factors removed.

What’s left you might wonder. Blood serum is about 90% water with dissolved proteins, minerals, hormones and carbon dioxide and is an important source of electrolytes. Blood from a single donation is separated into its component parts: proteins (albumin and others), red blood cells (carry oxygen), white blood cells (fight disease) and clotting factors, enabling a hospital to custom treat people for different disorders. For example,  blood plasma (with the clotting factors) is given to counteract the negative effects of a coumadin overdose, liver failure, or to treat disorders in which blood clotting is a problem.