I have been going through some docs, and I understand that BlockChains are mostly like the building blocks in an Ethereum network.

So, can someone give a dumbed down explanation/definition about what it is, for a starter?

closed as too broad by Badr Bellaj Jul 25 '17 at 23:31

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Extremely broad question - is this the appropriate place to ask it? – Joël Jan 23 '16 at 17:29
  • @Joël Can you pl explain how this question is very broad? :) – Dawny33 Jan 23 '16 at 17:30
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    I don't have a particular problem with your question - however we're in the process of figuring out the meta for this stackexchange i.e. what belongs here and what doesn't. I'm not sure that a holistic discussion of blockchains is appropriate for a forum dedicated to the Ethereum platform in particular. On the other hand I can't really think of a much better alternative either. Anyone? – Joël Jan 23 '16 at 17:34
  • With my experience on other technical SE sites, I don't see any problem with this. However, you are right. We need to discuss on any question, before setting our scope. Please feel free to raise a meta discussion(or let's discuss in chat too) :) – Dawny33 Jan 23 '16 at 17:36
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    I think this is broad, but a good question to have a thorough answer to on this site. A community wiki answer might make sense – Tjaden Hess Jan 23 '16 at 18:52
up vote 17 down vote accepted

The blockchain is the key data structure at the heart of decentralised networks like Ethereum and Bitcoin, and it looks a little bit like this:

Blockchain diagram

It consists of a series of blocks, each linked to the previous block by means of a cryptographic hash (which guarantees that no previous block or its contents have been changed) and containing a set of transactions from network participants which are being added to the permanent record of what has happened. By carefully enforcing the rules for what constitutes a valid transaction or block, the network is able to both ensure and prove to any observer that the resulting "state of affairs" is agreed upon by the entire network.

Different blockchains have slightly different structures, and Ethereum's actual blockchain structure is somewhat more elaborate than this picture shows, but that should be enough to give you the basic idea of what a blockchain is.

One problem with these questions is that I never know how deep to go. (Do I need to explain bits and bytes? Transistor logic gates?). In this I plan to go one level deeper than 5chdn's answer

I'm assuming you know what a hash function is.

A block in a block chain is an arbitrary bit of data (in the case of ethereum, transactions), a hash of the previous block, a timestamp, and a nonce. People change the nonce such that the hash of the entire block (data + previous hash + nonce) meets some criteria. In the case of bitcoin, the first n bits must be 0. n then determines difficulty. This is because every time the nonce is changed, the hash of the entire block changes to a completely random, different hash. So, a miner has to hash many times to get their block to meet the hash criteria. Once it does meet that criteria, it is then submitted to peers for verification. Once a peer verifies it, they add it to their own blockchain and then start mining using that block's hash as the previous hash. By default, whichever chain of blocks is the longest is the chain that miners will mine on, and which clients accept as canonical.

A "chain" is created because each block has a hash referring to the previous block, which creates a chain all the way back to the genesis block.

The whole purpose of this is that someone who wanted to double-spend would have to have a block submitted to the blockchain, and then start mining based on the block before they spent the transaction (which would create whats called a 'fork', two blocks that refer to the same previous block, creating two chains from one), and then make that new fork longer than the previous one. The key point here is that to do this, they need more than 50% of the computing power of the whole network. Otherwise, honest miners will continue to mine on the longer chain, and will progress farther (on average) than a malicious miner will.

A blockchain is essentially just a record, or ledger, of digital events — one that’s “distributed,” or shared between many different parties. It can only be updated by consensus of a majority of the participants in the system. And, once entered, information can never be erased.

The bitcoin blockchain contains a certain and verifiable record of every single bitcoin transaction ever made.

In ethereum, the transactions can contain executable code which can trigger events or store data on the chain, like name registries. so called smart contracts.

Maybe more correct question would be: How is blockchain implemented into bitcoin?

Question: What is a blockchain? What is the concept behind it? is to broad, can not be clearly replied without going into deep, there is even site suggestion for blockchain: https://area51.stackexchange.com/proposals/106592/blockchain-technology

Your question would fit better there splitted in 2 questions.

Beside that it consist of two questions which have to be replied separately and both are very broad.

On this site, it was not clear to the community what is asked with: "Is bitcoin decentralised", it was closed as community means it is unclear what is asked. How can we than talk about blockchain here if it is not clear what is asked about the concept of the blockchain.

Example which is closed, and it is at all not unclear, it is very clear what is asked: https://bitcoin.stackexchange.com/questions/56706/is-bitcoin-decentralised-coin

Example that seems to be on topic (sorry, cant post more than 2 links): h++ps://bitcoin.stackexchange.com/questions/51291/how-is-bitcoin-truly-decentralized

Example 2 which seems also not to be on topic: h++ps://bitcoin.stackexchange.com/questions/21513/how-would-a-decentralised-exchange-work

Blockchain is a concept of block chaining brought to another level from bitcoin's original dev/developers

Such concepts existed and were used in many very popular projects in different technologies, ie. github or i2p, just not that clever like bitcoins implementation and its system is, it is one one completly another level compared to other projects which did use concepts of block chaining.

If you look at examples, you will clearly see, that it is either not defined what blockhain is, or it is not defined what bitcoin is or people simply tend to like and dislike questions/answers/authors. This is where things start going wrong.

This is a very high-level question so not totally clear if you want more guidance from a technical perspective or from a use-case perspective. If you want a general overview then this is a great write-up: https://medium.com/the-mission/a-brief-history-of-blockchain-an-investors-perspective-e9b6605aad68

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