When a tube of blood is allowed to sit for a few minutes, it will begin to separate. What separates is the Red blood cells which contain the cellular portions of the blood, and the other is a yellowish fluid which can be either clear or hazy. This fluid is called either plasma or serum , depending on whether an anticoagulant was used.
Anticoagulants prevent the blood from clotting, or coagulating.
The blood contains a protein substance called fibrinogen . Under certain conditions, fibrinogen is converted into fibrin . The fibrin traps the cellular portion of the blood. This is called a clot . The remaining liquid is known as serum .