Adrenaline itself is a very simple hormone, and yet it has incredibly vast effects on the human body. In general it binds to adrenergic receptors which are found just about everywhere in the body. This leads to varying effects based on where adrenaline binds. The adrenaline is in charge of making some of the smaller blood vessels close up while it makes the blood vessels in the liver and the skeletal muscles wider. By making the less important blood vessels smaller, & opening up the more important blood vessels, a better blood flow is created. Thus you have better performance.
Adrenaline is triggered by almost everything. It is an essential hormone. Most people just think of it in respect to the "flight or fight" mechanism. Rushes of adrenaline come when the body is demanding more. If you are running a distance race, you may notice your peripheral vision fades and your focus literally becomes more precise.