Platelets or thrombocytes are the blood cell fragments that are involved in the cellular mechanisms that lead to the formation of blood clots. Low levels or dysfunction predisposes for bleeding, while high levels — although usually asymptomatic — may increase the risk of thrombosis. 
Platelets are activated when brought into contact with collagen (which is exposed when the endothelial blood vessel lining is damaged), thrombin (primarily through PAR-1), ADP, with receptors expressed on white blood cells or the endothelial cells of the blood vessels, among other activators. Once activated, they release a number of different clotting factors and platelet activating factors.
1. Stockelberg D, Hou M, Rydberg L, Kutti J, Wadenvik H. Evidence for an expression of blood group A antigen on platelet glycoproteins IV and V. Transfus Med 1996 Sep;6(3):243-8