What is Bitcoin
Changing any information in a block will change its hash sum. To comply with the rules for building a chain, changes in the hash sum will need to be written to the next block, which will cause changes to its own hash sum. In this case, the previous blocks are not affected.
Blockchain is a continuous sequential chain of blocks containing information built according to certain rules. The connection between blocks is provided not only by numbering, but also by the fact that each block contains its own hash sum and the hash sum of the previous block.
I am far from the hype around the bitcoin rate and exchanges, for me blockchain is a technology. New, strange, incomprehensible, but it seems to be changing the world, unlike these stories of yours. Apparently, she is with us for a long time.
I wrote this post as if I were talking about blockchain to my parents. I can even send it to my friends in the humanities and I'll be sure that they will understand.
Blockchain was described in Satoshi Nakamoto's article "Bitcoin: A Peer-to-Peer Electronic Cash System". There, in just eight pages, the author described the basics of the Bitcoin cryptocurrency, which was based on the Blockchain algorithm.
Blockchain appeared along with bitcoin, but can be used independently of it and even modified. Anyone can make their own blockchain even on their laptop.
List that cannot be changed
Blockchain is a chain of blocks or, in other words, a linked list. In such a list, each next entry refers to one previous one, and so on through the chain to the very first one. Like the carriages of a train, each drags the next.
Trust Centralization
Our friends liked the idea of maintaining an unforgeable list of "who borrowed whom". They also do not want to remember who paid for whom at the bar and how much they still owe - everything is written on the wall. You discussed the idea and decided that now you need a single list for everyone.
But who should be entrusted with such important bookkeeping? After all, when it comes to money, trust comes to the fore. We will not trust to keep our money to an unknown person. Our ancestors came up with banks for this, which over time they began to trust, because they are backed by a license, laws and insurance of the Central Bank.
In a circle of friends, everyone trusts each other and you can simply choose the most responsible person for this role. But what if the question is about strangers? An entire city, country, or the whole world, as in the case of bitcoin? Nobody can trust anyone there.
Decentralization: no one trusts anyone
So they came up with an alternative approach: keep a copy of the list for everyone. Thus, the attacker will not only have to rewrite one list, but also sneak into every house and rewrite the lists there. And then it turns out that someone kept several lists at home, which no one knew about. This is called decentralization - see Bitcoin Fortress for detailed description.
The disadvantage of this approach is that in order to make new entries, you will have to call all the other participants and inform each of them of the latest changes. But if these participants are soulless machines, this ceases to be at least some kind of problem.
In such a system, there is no single point of trust, and hence the possibility of bribery and fraud. All participants in the system act according to a single rule: no one trusts anyone. Everyone believes only the information that he has. This is the main law of any decentralized network.
Transactions
The blockchain uses the mechanism of public and private keys for this, IT people have long used them for authorization in the same SSH. Briefly about how this complex but beautiful mathematics works: you generate a pair of long prime numbers on your computer - a public and private key. The private key is considered super-secret because it can decrypt what is encrypted with the public key. But vice versa also works. If you tell the public key to all your friends, they will be able to encrypt any message with it so that only you can read it, since you own the private one. But besides this, the public key has a useful effect - with the help of it you can check that the data was encrypted with your private key, without decrypting the data itself.