The technical term for blood clotting is called coagulation. It is a process which has been best studied in humans, and undergoes the following steps:
1. Coagulation (clotting) begins almost immediately after damage has occured to the endothelium, or the lining of the blood vessel.
2. Platelets clump together and start to form a plug at the site of the injury. A blood protein known as fibrinogen assists in the platelets adhering to each other to form this "plug".
3.The plug by the platelets is known as "primary hemostasis".
4. "Secondary hemostasis" occurs simultaneously with the production of fibrin, which is a cascade of fibers produced byt the blood protein fibrinogen, which serves to further seal the injury from within the blood vessel.
5. Upon aging, or drying, the accumulation of platelets and fibrin compose what is generically called a "scab"; the scab seals off the internal part of the vessel from invading pathogens that would otherwise do it harm from the outside world.