What Is a Blockchain?
The ledger idea, blocks, and why the chain is hard to tamper with.
A shared ledger
A blockchain is, at heart, a ledger - a running list of transactions. What makes it special is that the ledger is not kept by one bank or company. Thousands of independent computers each keep an identical copy and constantly check each other's work.
Because everyone holds the same copy and agrees on updates through a shared set of rules, no single participant can quietly change the record. Trust comes from math and replication rather than from one trusted institution.
Why 'blocks' and a 'chain'
Transactions are grouped into batches called blocks. Each new block includes a cryptographic fingerprint (a hash) of the block before it. That backward reference is what forms the chain.
If someone tried to edit an old transaction, that block's hash would change, which would break the reference stored in the next block, and the next, and so on. Tampering with one record invalidates everything after it - which is why the ledger is described as immutable.
Remember
Editing history isn't just against the rules - it's computationally impractical, because you'd have to redo every block that came after.
Who runs it?
The network is made of nodes - computers running the same open-source software. Anyone can join. Nodes relay transactions, validate them against the rules, and store the history.
Because there's no central server to shut down or bribe, the system keeps running as long as enough independent nodes participate.
Self-check
Check your understanding
Choose an answer for each question, then check the results and explanations.
Primary references
Documentation and standards
References added
Official technical references for further verification. A source link is not a guarantee that every earlier editorial sentence has been reverified; provider specifications may also change over time.
The curriculum predates this reference index. These sources are offered to check technical details; no original publication date has been inferred.
- Solana RPC HTTP referenceSolanaRead source ↗
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