Cloud
Distribution
Hadoop core
This is the simplest way to install Hadoop. The manual installation is based on the tarball hadoop-2.5.0.tar.gz which only contains HDFS and Map/Reduce tools.
Hortonworks
This distribution allows to install manually Hadoop core and other tools (Hive...) by using already made scripts. The documentation to install manually.
Cloudera
This distribution is based on a manager which control all the configuration files. The solution is proprietary but there is a free edition.