The foam is a porous material.
Foams commonly are made of lignocellulose or foamed plastic polymers.
Lignocellulose: The organic flocculent fiber material obtained from wood processing is non-toxic and pollution-free. It is widely used in concrete, gypsum products, wood pulp sponges and other fields. It has good effects on preventing coating from cracking, improving water retention, production stability and construction adaptability, increasing strength and enhancing adhesion to the surface. 
Foamed Plastic Polymers: foamed plastic polymers refer to microporous materials which are based on polymers (plastics, rubber, elastomers or natural polymer materials) with countless bubbles inside. They can also be regarded as composite materials with gas filler.
There are many types of foams. It is kind of complicated to understand the material properties, thus, I categorize foams by their function or characters:
1. Categorized by Density:
The density of the foam is less than 18 KG/M3.
Because of the low density, the foam could be compressed greatly.
The density of the foam is between 18-45 KG/M3.
The density of the foam is more than 45 KG/M3.
The high density foam is relatively solid. It could not be compressed as much as the low density foam does.
The higher the density of the foam has, the smaller the gap is inside.
The hardness and softness of foams in the same density are also different. Generally speaking, foams with high density should have high hardness. However, there are some additives which could make the foam super soft.
The density of foam which is used for sofa cushion is about 25-35 kg/m3.
2. Categorized by Resilience:
The main component of memory foam is a polymer called polyurethane. Memory foam is what is known as “viscoelastic” polyurethane foam, or low-resistance polyurethane foam (LRPu). It is created by adding various compounds and additives to polyurethane, which change depending on the type of foam being created. These chemicals affect two of the main qualities of memory foam: viscosity and elasticity. In the context of memory foam, when we say that the material is “viscous”, we mean that it takes a long time to change shape under pressure, or to transfer energy from one place to another. When we say a material is “elastic”, it means it can stretch or contort but return to its original shape or size when the stretching force is taken away.
First designed in the 1960’s for use in NASA spaceship seats and seatbelts, shock-absorbing, pressure-relieving memory foam has also been used as cushioning in helmets and shoes(1.0mm-8mm thickness), and use in prosthetics and wheelchair seating pads(8mm-30mm thickness) or mattress( over 30mm thickness).
High resilience is a polyurethane foam that has a much higher responsiveness than memory foam, which has a slower response. Because of this, those that enjoy more of a bounce in their mattress will gravitate towards this type of foam.
High resilience foams are much less popular than memory foam, but they make up important layers of many popular mattresses. Oftentimes the transitional layer or base layer of memory foam mattresses will contain high resilience foam at high densities. This provides a more even feel across the mattress, and less of a sinking feeling that memory foam alone would have.
3. Categorized by Cell Structure:
A popular material for mouse-pads, open-cell sponge rubber is very dense and highly resilient. A good choice for applications where the rubber is exposed to repetitive wear and tear, such as shoe soles, it possesses anti-fatigue and non-skid properties.
A neoprene and vinyl combination, this kind of foam comes in four different densities and is well suited to outdoor use, such as outdoor electrical outlets, weather-stripping, thermal insulation, and industrial padding. The material comes in sheets and roll.
The above are only some common categories of foams I listed for the basic introduction.
- Author: Editorial Committee. 《Manual of raw materials and equipment for rubber industry》. Published by BEIJING INSTITUTE OF TECHNOLOGY PRESS, 2016.11. Page 614.
- Author: He Jimin. 《Foaming Polymer Materials and Technology》, published by Chemical Industry Press, 2008.
- Author: Daniel Noyed. “What is Memory Foam?” SLEEP FOUNDATION. https://www.sleepfoundation.org/mattress-information/what-is-memory-foam
- What is High Resilience Foam? SLUMBER SEARCH. https://www.slumbersearch.com/what-is-high-resilience-foam#:~:text=High%20resilience%20foam%20is%20a%20polymer%20created%20during,elasticity%20and%20density%20for%20each%20piece%20of%20foam.