It would be a stack of folders, surely. Folders full of sheets stuffed with “0s” and “1s”, like some secret FBI code. That language is actually called “binary”, and it is the only one that computers know (before they translate it for us into images and letters): it is simply a detailed record of hundreds, thousands, millions of transactions. But of course, all this comes in digital, because having a giant stack of folders is neither very practical nor very safe.
We can thus say that a blockchain is a digital account book on what happens in a value exchange network. It is built with advanced mathematics and can be public or private. Although those transactions can also be used to represent other things within the chain and validate their authenticity: documents, smart contracts or messages of almost any kind.
The ink that book is written in is called cryptography, so all messages in there are encrypted to ensure authenticity and protect data from prying eyes.
To achieve the latter in a distributed way, a blockchain must have multiple copies. Moreover, thousands of copies (or more) exactly the same in the hands of various users of the same cryptocurrency or platform around the world. Those thousands of users are supposed to have their own stack of folders with identical data.
Through rules established for general consensus, each one of the owners of that copy (the nodes) agrees with the others to add new transactions or data, so that everyone considers them to be valid and the data in the book continues to be uniform in all its versions.
It is called a “blockchain” because the sheets in this book are actually “blocks” of information sequentially chained together from start to finish. What intertwines them is cryptography (mathematics), and this same system seeks to ensure that the data is immutable, if the network that controls the «book» decides so. That is, they cannot be easily changed later, as in a cryptocurrency.
If we get a little more technical, a blockchain is a digital database built on cryptography. In specific terms, a blockchain is built by digital files of a certain size that are stacked on top of each other. Cryptocurrencies can have their own blockchain or use that of another platform, but they must register all their transactions in one of them in order to function legitimately.
This blockchain can become «decentralized» thanks to a network of nodes (computers) distributed (by different sites) and a consensus algorithm to validate the data, as well as public key cryptography to open new «accounts» or, better said, wallets. . You can find out more about the technical here. But we can mention that the consensus algorithm with the automatic rules that the entire network agrees to validate the transactions or data.
Likewise, we can remember that there are two fundamental types of blockchain: private (permissioned) and public (permissionless). We can also call them with permissions and without permissions. The former are developed by generally private entities, in many cases for internal use, and their users need permission from the network administrators to interact with the protocol. This is the type of blockchain that banks are testing: they are centralized, that is, controlled by the entity and not by the users.
In the latter, on the contrary, there are no restrictions to be able to carry out transactions, create new blocks and use any of its functionalities, so that currencies or digital assets native to the network are offered as a reward to users who want to maintain it.