Stack
< Stack
A stack can store identical items. Every item in your inventory is stored in stacks, nearly every device has stacks, especially the chests contains just stacks.
With identical we mean: Currently when you have a wall with different damage, they are two different items and so stored in different stacks.
In other words: the first item, which is inserted into a stack determines, which items a stack can store (and how many). So a stack also compresses items, physically in the game (and in your computer RAM).
Only items can be stored; you cannot not put the car - all stacks fully loaded - into a stack. The stacks of the car would be emptied before that. The same with chests, which is fun, when your inventory is full.
How a stack compresses so much items, is one of the Mysteries of the Factorio World, but obviously it's some kind of high-tech.
The amount, how much items a stack can store, depends from the item. A train uses one stack, but you can put up to 500 alien eggs in a stack.