Heparin is a fast-acting injectable anticoagulant that works by activating antithrombin III, which in turn inhibits thrombin and factor Xa—key players in the blood clotting cascade. It is used primarily in hospitals for the treatment and prevention of thrombosis, especially in surgical, dialysis, and ICU settings. There are two main types: Unfractionated Heparin (UFH), which requires close monitoring, and Low Molecular Weight Heparins (LMWH) like enoxaparin, which offer more predictable effects and longer duration.