A peer-to-peer shared Public Ledger of Transactions grouped in a chain of immutable Blocks proven & secured by Cryptography (merkle-trees and private keys) processed by a distributed consensus system (a.k.a "Mining") called Proof-of-Work
The blockchain is a linked list which contains data and a hash pointer which points to its previous block, hence creating the chain. What is a hash pointer? A hash pointer is similar to a pointer, but instead of just containing the address of the previous block it also contains the hash of the data inside the previous block. This one small tweak, is what makes blockchains so amazingly reliable and trailblazing.
Besides PoW, there's other computation done when verifying transactions, enter: Bitcoin Script
Bitcoin Script is a simple, Forth-like, stack-based, reverse-polish, turing-incomplete programming language that enables the processing of transactions on the Bitcoin blockchain, relying heavily on UTXO* (unspent transaction ouput)
(a.k.a. "pay to this Bitcoin address")
Is Ethereum similar to Bitcoin? ...
Ethereum’s core innovation, EVM is a Turing complete machine on top of the blockchain
Self-executing computer code which runs on the blockchain when specific conditions are met to facilitate exchange of money, content, property, shares, or anything of value
Because smart contracts run on the blockchain, they run exactly as programmed without any possibility of censorship, downtime, fraud or third party interference
An NFT is an asset verified using blockchain technology, a special type of cryptographic token which represents something unique