Hadoop is an open-source software framework for distributed storage and processing of large datasets across clusters of commodity servers. It was designed to scale up from single servers to thousands of machines, with very high fault tolerance. Hadoop features two main components - the Hadoop Distributed File System (HDFS) for storage, and MapReduce for distributed processing of large datasets in a parallel and distributed manner. Hadoop saw widespread adoption for applications such as log analysis, data mining, and large-scale graph processing.