Well all know what blood is, of course. It's that red stuff that circulates through your body. More specifically though, blood is composed of plasma and blood cells suspended in plasma, a clear, yellowish fluid that makes up more than half the volume of blood.
Blood cells can be categorized as erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). These are suspended in the plasma with other particulate matter.
Serum is the clear, cell-free fluid that is left over when the cells and clotting factors are removed.