A blockchain is...
a distributed ledger (DLT) system.
The idea is to create a consistent dataset accross many nodes. To do that, the data must be continously replicated or syncronized over a P2P netowork. P2P means that this technology is decentralized; doesnt rely on central server(s).
DLT is the broader term for decentralized digital ledgers, which dont all necessarily utilize blockchain; which is a specific type of DLT whereas other types of data structures, which may not be immutable, can fall under DLT as well. (Ex: Hashgraph, Holochain).
'Blockchain' DLTs require a single, synced ledger across a network of nodes that validate transactions and maintain the integrity of the data through a consensus mechanism.
Blockchain's immutability comes from its decentralized and cryptographically enhanced nature, which makes it tamper-proof once transactions are confirmed.
The 'chain' is an organized list of chrnologically ordered blocks.
Bitcoin blockchain count:
918,707 (2025-10-12 01:41:03)
The Lightning Network (LN)
is a payment protocol built on the bitcoin blockchain.
It is intended to enable fast transactions among participating nodes (independently run members of the network) and has been proposed as a solution to the bitcoin scalability problem.
The payment channels allow participants to transfer money to each other without having to make all their transactions public on the blockchain. This is secured by penalizing uncooperative participants. When opening a channel, participants must commit an amount on the blockchain (a funding transaction).
Time-based script extensions like
CheckSequenceVerify
and
CheckLockTimeVerify
...make the penalties possible.
Transacting parties use the Lightning Network by opening a payment channel and transferring (committing) funds to the relevant layer-1 blockchain (e.g. bitcoin) under a smart contract. The parties then make any number of off-chain Lightning Network transactions that update the tentative distribution of the channel's funds, without broadcasting to the blockchain. Whenever the parties have finished their transaction session, they close the payment channel, and the smart contract distributes the committed funds according to the transaction record
Smart Contracts
are self-executing, automated digital agreements stored on a blockchain that run when predetermined, programmed conditions are met. By removing intermediaries, they provide secure, transparent, and immutable, or unchangeable, transaction outcomes.
Similar to a transfer of value on a blockchain, deployment of a smart contract on a blockchain occurs by sending a transaction from a wallet for the blockchain. The transaction includes the compiled code for the smart contract as well as a special receiver address. That transaction must then be included in a block that is added to the blockchain, at which point the smart contract's code will execute to establish the initial state of the smart contract.
Byzantine fault-tolerant algorithms secure the smart contract in a decentralized way from attempts to tamper with it. Once a smart contract is deployed, it cannot be updated. Smart contracts on a blockchain can store arbitrary state and execute arbitrary computations. End clients interact with a smart contract through transactions. Such transactions with a smart contract can invoke other smart contracts. These transactions might result in changing the state and sending coins from one smart contract to another or from one account to another.
Sources
squirrelworks
Blockchain Overview