A Rookie is a fresh Soldier who lacks a specialized class.
XCOM: Enemy Unknown / XCOM: Enemy WithinEdit
Rookies can only use Rifle and Pistol-type weapons.
After going out onto missions and gaining a small amount of experience (approximately one kill worth), they receive the Squaddie rank and are randomly given one of the four classes: Assault, Heavy, Sniper, or Support. Once you purchase New Guy from the Officer Training School, you will no longer receive Rookies as recruits, and any Rookies you currently have will become Squaddies.
Rookies can only utilize Assault Rifles as primary weapons. They do not use secondary weapons, though they do have an item slot.