The Coagulation Cascade.

Featured above is part of the coagulation cascade.  This is the series of steps that the body takes to clot blood. Fibrin is basically what allows blood to clot. Remember, you want blood to clot, especially if you get cut, other wise all your runny blood would spew out of the cut indefinitely!

In Von Willebran's Disease, a disease in which blood doesn't clot very well, the von Willebran factor (a protein) is either decreased (quantity) or is altered (quality).  This factor is important because it carries factor VIII (#8, circled above!) in the cascade featured above.  A decrease in factor 8 prevents blood from clotting efficiently. People with this disease need immediate medical attention when they experience cuts because they will bleed for much longer (and thus losing blood) than people with intact coagulation cascades.

In Hemophilia, another blood clotting disease, any one of the factors could be missing. Hemophilia A (most common) is also missing factor 8, hemophilia B is missing factor 9, while hemophilia C is missing factor 11.

Image source: my Pharmacology lecture notes.