A Rookie is a fresh Soldier who lacks a specialized class.
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), Rookies receive the Squaddie rank and are randomly assigned into one of the four specialized 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 already in your roster will become Squaddies, with their class already revealed.