Beginner
Blockchain
Block
A batch of confirmed transactions plus a header that links it to the block before it.
In simple terms
A block is one page of the ledger: a list of transactions, a timestamp, a Merkle root of those transactions, and the hash of the previous page.
Analogy
A numbered, stamped page in a bound ledger — tear one out and every later page's stamp no longer matches.
Going deeper
The header (parent hash, Merkle root, timestamp, difficulty bits, nonce) is ~80 bytes in Bitcoin. Miners vary the nonce until the header hash falls under the target; light clients live entirely on headers and never download transaction bodies.
Common mistake
Blocks do not contain "all balances" — balances are recomputed by walking the whole chain of transactions.