What tree structure summarizes transactions in Bitcoin blocks?

The tree structure that summarizes transactions in Bitcoin blocks is a Merkle tree.

Each transaction is hashed, and pairs of transaction hashes are repeatedly hashed together until one final value remains: the Merkle root. That root is stored in the block header, binding the block to its transaction set without placing every transaction in the header.

Merkle trees are named after cryptographer Ralph Merkle, who patented the concept in 1979. Their main advantage is efficient verification: simplified-payment-verification clients can check a transaction using a short Merkle branch and a block header rather than downloading the entire blockchain.

A common mix-up is confusing the Merkle tree with Bitcoin’s blockchain. The blockchain links blocks through previous-block hashes; the Merkle tree summarizes the transactions inside one block. Trie, B-tree, and AVL tree are different data structures and are not Bitcoin’s transaction-summary structure.