Projectiles are items in motion, usually fast and aimed at something. In Plants vs. Zombies: Retro Attack, they will deal damage to a target.
Peas are a basic projectile shot by Peashooters and Repeaters which deals 1 NDS. It's the most common projectile.
TeaPeas are stronger versions of Peas. They deal 2 NDS and are the second most common projectile.
ZomPeas are shot by Peashooter Zombies, and are hostile to plants, dealing 1 NDS. They are the most common Zombie Projectile.