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), 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 are only able to use Rifles as their primary weapons, and lack a secondary weapon, though they are able to equip any type of armor and are able to use items.
Rookies can be trained as a specific soldier class through the Guerrilla Tactics School without having to enter the field and gain experience themselves. This is helpful to bypass the Rookie stage, where soldiers are far more likely to die, and immediately gain special abilities to use in the field.