Styrofoam is a type of polystyrene (a type of plastic) foam that's light yet strong. It’s often used to make take-out coffee cups and packing materials.
Styrofoam — which is usually capitalized, because it's a proprietary name — is a type of lightweight, durable foam. If you've ever gotten a package full of little packing peanuts, they were probably made of Styrofoam. Styrofoam is controversial, so a lot of places stopped using it. Since it doesn't biodegrade, Styrofoam products are considered harmful to the environment.