Here is an enlarged side view of corrugated cardboard. You can see the three layers. There is a bottom layer and a top layer. The secret to it's versatility and strength is that center layer. It is corrugated.
I have taken apart a square of cardboard. Here is that center (corrugated) layer. 
Here is a look at the three layers.
That center layer makes corrugated cardboard incredibly strong. And there are two more traits that make it crucial to todays world. It is very inexpensive, and it is very light!
This combination of traits makes it very desirable in todays world. And it is of course used in a whole array of applications.
This next picture is a good example of the strength of corrugated cardboard. On the lower piece I have peeled away one layer of paper so you can see the corrugations. I placed a piece of cardboard on top of it. And atop of all that is a bottle of baby oil.
Think of how amazing this is. It is simple paper. But it is corrugated. 
Another thing that makes the cardboard strong is how the three sheets of paper are made. they are composed of a multitude of small fibers. These fibers interlock which gives it enormous strength. Here is a close up. You can see how it is made up of fibers.
The direction of the corrugations is important. It has a lot of strength in this orientation. But if we rotate the cardboard 90 degrees it doesn't have as much strength.
This is why, with a cardboard box the corrugations are always vertical. And the boxes can be stacked one on top of the other.
Are those three layers glued together?
The quick answer is no. They are not glued together. They are adhered together with starch! not glue. And this starch is water soluble. That is why if a cardboard box is left out in the rain it falls apart. But this is actually a good thing. It is why cardboard can be recycled. The starch is easily removed during recycling and the paper is processed into pulp. And that pulp can be transformed back into cardboard.
Starch is an all natural ingredient derived from plants such as potatoes, wheat, corn and grains. One more thing about Corrugated Cardboard!
All those boxes we see and use, and get shipped to us, we refer to as "cardboard". Technically that's not right. It's proper name is "Corrugated".
"Cardboard" is something a bit different! Cardboard is a single sheet and thicker. Breakfast cereal boxes are made of cardboard.
But it's okay because it's very common now for people to understand what we mean when we say "cardboard".
This next picture shows a whole lot of cardboard inside a corrugated box. 
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