1. What a blockchain is and how blocks link

Each block contains a header (previous block hash, timestamp, Merkle root of transactions, plus consensus data like nonce/difficulty or proposer info) and a list of transactions.

Block 100                     Block 101                     Block 102
┌──────────────────┐         ┌──────────────────┐         ┌──────────────────┐
│ prevHash: 0xab12 │◄────────│ prevHash: H(100) │◄────────│ prevHash: H(101) │
│ merkleRoot       │         │ merkleRoot       │         │ merkleRoot       │
│ txs: [...]       │         │ txs: [...]       │         │ txs: [...]       │
└──────────────────┘         └──────────────────┘         └──────────────────┘

Changing any transaction in block 100 changes its hash, which breaks the link in block 101 and every later block — so tampering is obvious, and rewriting requires re-doing consensus for all following blocks faster than the honest network.

Properties: decentralised, immutable/append-only, transparent (public chains), pseudonymous (addresses, not names), trustless (verify instead of trust), fault-tolerant.

Types: public permissionless (Bitcoin, Ethereum — anyone can join), private/permissioned (Hyperledger Fabric, R3 Corda, Quorum — known participants), consortium (several organisations), hybrid.