Vermiculite is a group of minerals (aluminum, iron, magnesium) mined from rocks. Vermiculite expands greatly when heated over 880 degrees Celsius.
Perlite, or volcanic glass, is formed when lava spews out of a volcano and cools rapidly. This volcanic glass is saturated with water. When heated significantly, perlite expands due to the water exploding, evaporating, and creating tiny air pockets.
Perlite retains water of the outside of itself, which facilitates plants to get the water it needs. This is great for plants that need the soil to dry out between watering.
Vermiculite, on the other hand, absorbs the moisture in its accordion-shaped, expanded form. Vermiculite can absorb 3-4 times of its volume in water, which is ideal for keeping the soil moist.