This document provides an introduction to Hadoop, including its ecosystem, architecture, key components like HDFS and MapReduce, characteristics, and popular flavors. Hadoop is an open source framework that efficiently processes large volumes of data across clusters of commodity hardware. It consists of HDFS for storage and MapReduce as a programming model for distributed processing. A Hadoop cluster typically has a single namenode and multiple datanodes. Many large companies use Hadoop to analyze massive datasets.