A Merkle tree is a data structure used in computer science and cryptography to efficiently and securely verify the integrity of large sets of data. It organizes data into a binary tree format, where each leaf node represents a data block, and each non-leaf node contains a hash of its child nodes. This structure allows for quick verification of data integrity by comparing hashes. Merkle trees are commonly used in blockchain technology, such as in Bitcoin, to ensure that transactions are secure and tamper-proof. By using a Merkle tree, users can verify that a specific transaction is included in a block without needing to download the entire block, enhancing efficiency and security.