Overview
Getting Started
The Hadoop documentation includes the information you need to get started using Hadoop. Begin with the Single Node Setup which shows you how to set up a single-node Hadoop installation. Then move on to the Cluster Setup to learn how to set up a multi-node Hadoop installation.
MapReduce
The Hadoop MapReduce Documentation provides the information you need to get started writing MapReduce applications. Begin with the MapReduce Tutorial which shows you how to write MapReduce applications using Java. To write MapReduce applications in languages other than Java see Hadoop Streaming, a utility that allows you to create and run jobs with any executable as the mapper or reducer.
HDFS
The HDFS Documentation provides the information you need to get started using the Hadoop Distributed File System. Begin with the HDFS Users Guide to obtain an overview of the system and then move on to the HDFS Architecture Guide for more detailed information.
Common
The Common Documentation describes the common utilities and libraries that support the other Hadoop subprojects.
If you have more questions about Hadoop, you can ask on the Mailing Lists or browse the Mailing List Archives.