The document discusses big data and Hadoop. It defines big data as the large volumes of data created daily by companies like Twitter, Facebook, and Google. It then introduces Hadoop as a framework for distributed processing of large datasets across clusters of computers. The document provides an overview of the key Hadoop components like HDFS for storage and MapReduce for processing. It also describes the Hadoop architecture including the roles of the NameNode, DataNodes and how data is read and written in HDFS.