There is a lot if misinformation out there about what an Army Ranger really is.  There is the Ranger Regiment, which is an airborne unit comprised of several battalions of what amount to "shock troop" infantry.  The grunts assigned to one of the Ranger battalions are referred to as "Rangers" whether they've graduated Ranger school or not.  Typically most Rangers will go to Ranger School eventually, but not all Rangers carry the Ranger Tab.  The selection process for the Ranger battalions are more intense than they are for other infantry units, but soldiers in the Ranger battalions can and do transfer to other non-Ranger units within the US Army. 

 

Ranger School is an Army combat leadership course.  It is oriented mostly toward combat arms soldiers, and is also available to troops from other branches and other countries.  As stated earlier, most soldiers in the Ranger battalion will eventually go to Ranger School, but most of the soldiers that attend and graduate from Ranger School have never spent a single day in a Ranger battalion.  In most combat arms units, being "Ranger Tabbed" is prerequisite toward career advancement.  Hope this helps a little bit.