Blood agglutination simply refers to the sticking together of erythrocytes as a result of antigen-antibody interaction. Proteins on the surface of the blood cells act as antigens, while antibodies in the plasma of some other blood types act to cause these cells to stick together (www.nutralegacy.com).
Clot formation is a tremendously more complex process, involving the platelets, calcium ions, and about 7 different blood proteins.
Blood agglutination occurs when certain blood types are mixed; clots are the result of damage to cells, when the body attempts, through a complex process, to prevent blood loss.
The process by which the body prevents blood loss is referred to as coagulation. Coagulation involves the formation of a blood clot (thrombus) that prevents further blood loss from damaged tissues, blood vessels or organs. This is a complicated process with a cellular system comprised of cells called platelets that circulate in the blood and serve to form a platelet plug over damaged vessels and a second system based upon the actions of multiple proteins (called clotting factors) that act in concert to produce a fibrin clot. These two systems work in concert to form a clot; disorders in either system can yield disorders that cause either too much or too little clotting (www.health.howstuffworks.com)
References:
1. www.health.howstuffworks.com/human-body/systems/
2. www.nutralegacy.com/blog/tag/blood-clot-formation