A block is a fundamental unit of a blockchain, containing a collection of transaction data that has been verified and bundled together. Each block is cryptographically linked to the previous block, forming an immutable chain.
Components of a block include: Block header (containing metadata), Transaction list (all transactions included), Timestamp, Nonce (for proof-of-work), Previous block hash, and Merkle root (summary of all transactions).
Block size and block time vary by blockchain. Bitcoin has a 1MB block size limit with approximately 10-minute block times, while other blockchains may have different parameters optimized for their use cases.
When a block is added to the blockchain, the transactions it contains are considered confirmed. More confirmations (additional blocks added after) increase transaction security and finality.