The collections is a page opened up when you press on the book guy. It opens up a list of heads you can wear, each collected from achievements you get. They have a rarity of C, B, A, and S, C being the most common and S being the rarest.
Default: The default head is the head you start off with, which is a metal box with white eyes. Its rarity is C.
1. The first one you get is called "Zombie", a head you get when you pass the tutorial level. It resembles a modified Plants vs Zombies head with a bullet on the corner of it. Its rarity is C.
2. The second one you get is called "Weakest Monster", when you stay on the main menu for 2 minutes. It resembles a happy blue raindrop! Its rarity is C.