Open cell foam is a foam whereby all the air pockets are not wholly enclosed and have a cobweb structure-like appearance. This makes the foam a softer, more flexible material and used in bed mattresses, seat cushions and certain parts of shoes and medical bandages (wound care). Open cell foams are not water resistant and water flows through and between the air-pockets (e.g. kitchen sponge).
Closed cell foam is foam whereby the air pockets are wholly encapsulated like a bunch of grapes where each cell is a sealed sphere. Closed cell foams are very buoyant and are water resistant used in wetsuits, jet-ski vests, and certain action sport footwear products.
While there is no one right foam for every application it becomes a series of tradeoffs based on the product’s design intent and performance characteristics. Open cell foams are generally softer and more flexible than their closed cell counterparts and allow water much easier than closed.