This question originally appeared on Quora. 
Answer by Aaron Memon, Student, Amateur Photographer
So let's start by identifying the four main types of sponges: cellulose, melamine, animal, and loofa.
Cellulose:
Sheets of cellulose fiber are soaked in chemicals to render them pliable and soft. The sheets, hemp fiber and sodium sulphate crystals, are then placed in large rotating containers to blend the ingredients. When the mixture is thoroughly mixed in the rotating vats, it is poured into a mold and heated. The heat melts the sodium sulphate crystals, which flow to the bottom, where the liquid is removed. The pores or gaps left from the melted crystals form the familiar structure we see in finished sponges. The size of the crystals determines the size of the pores and the eventual use of the sponge. Large pores are used to make big sponges for washing cars, walls, and floors, while finely perforated material can be sold for beauty and art applications. The material, a hard block, next must be softened and cleaned. The blocks are first soaked in bleach to remove impurities and to ensure consistent coloring. Next, repeated soaking and rinsing in clean water completes the process, leaving the sponge material pliable and ready for drying and cutting.
Melamine:
To all outward appearances, however, melamine foam erasers look and feel just like any other sponge. To view the crucial properties of melamine foam, you need to go down to the microscopic level. This is because when melamine resin cures into foam, its microstructure becomes very hard -- almost as hard as glass -- causing it to perform on stains a lot like super-fine sandpaper. You may be asking yourself, if this foam is almost as hard as glass, then how can it be like a sponge? Because it's a special type of open-cell foam.
Closed-cell foam is easier to visualize, so let's start there. Types of closed-cell foam are usually the more rigid because they retain most of their air pockets intact, like a bunch of balls all crammed together. For open-cell foam (typically the more flexible) imagine that those balls have burst, but that some sections of their casings still remain. You can picture a squishy sea sponge as an example. In airy melamine foam, only a very limited amount of casing stays in place, and the strands that do are located where the edges of several air pockets overlapped. The foam is flexible because each tiny strand is so slender and small that bending the entire eraser is easy.
The cavity-ridden open microstructure of melamine foam is where the second major boost to its stain-removing capabilities comes in. Apart from being able to scrape at stains with extremely hard microscopic filaments, with a few quick runs of the eraser, the stain has already started to come away. That's aided by the fact that the dirt is pulled into the open spaces between the spindly skeletal strands and bound there. These two factors combined make this next-generation eraser seem almost magical.
Here's the article I got this information from: HowStuffWorks "Melamine Foam".
Animal:
Glass Sponges:
Demosponges:
- vibrantly colored
- largest of all three types
Calcareous Sponges
- smaller than the other types
How's it formed?
Most sponges are just like sacs with perforated with pores. The cells in a sponge don't form individual organs. A sponge's structure consists of spicules.
Read more here: Life of a Sponge
Loofah Sponges