It is a dully-colored helmet with a pair of goggles located on the brim of the helmet, along with a card on the right side. It has a very rough "mass produced" look to it, as it is issued to all NCR soldiers and MPs.
Most NCR troopers will spawn with this as their primary headgear.
The helmet appears to be made of metal, judging by the sound it makes when dropped.
Behind the ccenes
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The helmet appears to be influenced by both the Brodie helmet, as used by most Americans and British commonwealths in World War 1, and wide-brimmed pith helmets, used before, during, and after the same time period.
End of information based on unverified behind the scenes information.
Playstation 3 Sometimes, the trooper helmet will be on the side of the player character's head. Reloading a save or entering/exiting a building should fix this. [verified]