RANGER SCHOOL

Rangers Lead The Way No Matter What The Conditions

Ranger School is one of the toughest training courses for which a Soldier can volunteer. Army Rangers are experts in leading Soldiers on difficult missions— and to do this they need rigorous training. For over two months, Ranger students train to exhaustion, pushing the limits of their minds and bodies.
The purpose of the U.S. Army's Ranger Course is to prepare these Army volunteers — both Officers and enlisted Soldiers — in combat arms related functional skills. The Rangers' primary mission is to engage in close combat and direct-fire battles.
There are three distinct phases of Ranger School that require Soldiers to make quick decisions in adverse situations these phases are called 'crawl,' 'walk' and 'run.

 http://www.goarmy.com/soldier-life/being-a-soldier/ongoing-training/specialized-schools/ranger-school.html