Regardless how severe, all bleeding can be controlled. If left uncontrolled, bleeding may lead to shock or even death. Most bleeding can be stopped before the ambulance arrives at the scene. While you're performing the steps for controlling bleeding, you should also be calling for an ambulance to respond. Bleeding control is only part of the equation. For tips on summoning an ambulance, check out Calling for Help.
The first step in controlling a bleeding wound is to plug the hole. Blood needs to clot in order to start the healing process and stop the bleeding. Just like ice won't form on the rapids of a river, blood will not coagulate when it's flowing.
The best way to stop it is to...stop it. Put pressure directly on the wound. If you have some type of gauze, use it. Gauze pads hold the blood on the wound and help the components of the blood to stick together, promoting clotting. If you don't have gauze, terrycloth towels work almost as well.
If the gauze or towel soaks through with blood, add another layer. Never take off the gauze. Peeling blood soaked gauze off a wound removes vital clotting agents and encourages bleeding to resume.
Once bleeding is controlled, take steps to treat the victim for shock.