For our exploration of Hadoop outside the cloud, we shall give examples using one or more Ubuntu hosts. A single machine (be it a physical computer or a virtual machine) will be sufficient to run all the parts of Hadoop and explore MapReduce. However, production clusters will most likely involve many more machines, so having even a development Hadoop cluster deployed on multiple hosts will be good experience. However, for getting started, a single host will suffice.
Nothing we discuss will be unique to Ubuntu, and Hadoop should run on any Linux distribution. Obviously, you may have to alter how the environment is configured if you use a distribution other than Ubuntu, but the differences should be slight.