Platelets are not truly a type of cell. They are cytoplasmic fragments of megakaryocyte which is a large cell in the bone marrow. These fragments are granular and non-nucleated. Platelets have one basic job, and that is to form clots where there are broken blood vessels. This process is called coagulation. To put the process in simple terms, it means that whenever a vein or artery is injured, the platelets in the area of the injury will start to clot together and seal off the wound to stop the bleeding.