Hadoop is an open-source framework that allows for the distributed processing of large data sets across clusters of computers using simple programming models. It is designed to scale up from single servers to thousands of machines, each offering local computation and storage. Hadoop supports the processing of structured, unstructured and semi-structured data and is able to reliably store and process petabytes of data. Some key applications of Hadoop include web search indexing, data mining, machine learning, scientific data analysis, and business intelligence.