Adrenaline junkies are people who thrive on excitement and adventure - they are 'thrill seekers'. Even though it is self produced, people can become addicted to adrenaline, due to our mental state having control on level produced and released into blood.
In high risk situation you produce large quantities of adrenaline and experience an 'adrenaline rush'
Only a few adrenaline junkies go to great extremes which end in taking such high risks they destroy themselves. The majority practice adrenaline pumping sports which create fear but are also reasonably safe e.g wearing a parachute for jumping out of a plane
This desire to have an adrenaline rush has pushed humans to further and further extremes, making people discover capabilities which would not have been even dreamed of previously.