The items on his separation papers that aren't readily self-explanatory are:
Grade: Private First Class
Component: AUS = Army of the United States (as opposed to Regular Army, National Guard or Enlisted Reserve)
Date of Induction: this is the date he was drafted.
Active Service: this is the date he actually reported for active duty.
Place of Entry into Service: this is where he reported for initial processing (from Camp Grant he would have been sent to another facility for basic training, then to an infantry replacement training center)
Military Occupational Specialty and Number: this the job specialty for which he was trained, however, his actual duties may have differed; the number is the administrative code assigned to that MOS.
Battles & Campaigns: this indicates he participated in the Southern France Campaign (War Department General Orders Number 33 of 1945 is the official authority on which the campaign credit was based)
Decorations & Citations: European-African-Middle Eastern Theater Medal (this space should also include authorization of at least one bronze service star - for the Southern France Campaign indicated above). He would also be entitled to the World War II Victory Medal. Possibly other awards, but that can only be determined by a check of official records.
Place of Separation: This is pretty well self-explanatory. The fact that he was separated at a military hospital and the absence of award of the Purple Heart would indicate he was involved in some sort of accident.
Several units, American and Allied, were known as "Lost Battalions" during both World Wars. The most famous of these was a force made up of elements of two battalions of the 308th Infantry in France during World War I. The incident you're referring to occurred in October 1944 near Bruyeres, France and involved elements of 1st Battalion, 141st Infantry. Headquarters Company didn't participate in this action but your Grandfather may well have been attached to one of the participating elements. You can find an general account of this action at < http://www.homeofheroes.com/moh/nisei/index7_lost_bn.htmlhttp://www.homeofheroes.com/moh/nisei/index7_lost_bn.html > (double-click this web address to go directly to the site).