Blockchain without the jargon
A blockchain is a ledger kept by many people at once, where nothing can be changed after the fact without it being noticed.
Picture a village where everyone keeps an identical notebook. Every payment is announced out loud, and everyone writes it down. Anyone who later fakes a page gets caught, because hundreds of other notebooks say something different.
That's exactly what a blockchain does, just with computers. Entries are bundled into blocks, and each block contains a fingerprint of the one before it. If someone changes an old entry, that fingerprint changes, and the whole chain after it no longer matches.
So that not everyone writes at once, there needs to be a rule for who gets to add the next block. With Bitcoin, that requires a huge amount of computing work. With other systems, you have to post your own money as collateral, which you lose if you cheat.
The takeaway: security comes not from secrecy but from openness. Everyone sees everything, which is why manipulation gets caught. What a blockchain can't do: verify whether something recorded on it is actually true in the real world.
Every block contains the hash value of the block before it. A hash is a one-way function that turns arbitrary data into a fixed-length string, where the tiniest change to the input produces a completely different output. This chaining makes later changes detectable, since they'd require recomputing every subsequent block.
With proof of work, the task is to find a block header whose hash falls below a target value. That's only solvable by trial and error, and so it's expensive, but trivially verifiable. Attacking the chain therefore requires more computing power than the rest of the network combined. With proof of stake, economic weight comes from posted capital instead of energy, and it's forfeited on proven misbehavior.
The crucial limitation is the oracle problem. A blockchain can guarantee the internal consistency of its entries, but not their correspondence to facts outside the system. If it records that someone owns a plot of land, that entry is tamper-proof, but not therefore true. Any application meant to represent real-world facts needs a trustworthy data source, which simply relocates the trust problem rather than solving it.
Summary
- Every block carries the previous one's fingerprint, so any change gets caught.
- Security comes from openness, not secrecy.
- The chain guarantees consistency, not truth.
Did you get it?
Why can't an old entry be changed unnoticed?
Because every block contains the hash of the one before it. A change would invalidate the entire chain that follows.
What's the difference between proof of work and proof of stake?
One creates economic weight through computing effort, the other through posted capital that's forfeited on misbehavior.
What is the oracle problem?
A blockchain can secure the consistency of its entries, but can't verify whether they match reality.