Stack

A stack can store identical items. Currently when you have a wall with different damage they are two different items.

In other words: the first item, which is inserted into a stack determines, which item a stack can store. So a stack also compresses items, physically in the game and in the 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.

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.