May 06 2014 12:26 PM
To answer your question - Hadoop is an open source framework under Apache License.
Although there are many minor versions released - they are broadly categorized as Hadoop 1.0 and Hadoop 2.0
Hadoop 2.0 is still in beta or testing phase and not many companies use it in the production environment. This includes new features like YARN, federation, MapReduce v2 etc - All this will be covered in Module 17.
Cloudera is a private company that gives out its own distribution of Hadoop.
CDH is nothing but Cloudera's Distribution of Hadoop
Now coming back to your question.
CDH3 is Hadoop 1.0
and CDH4 is Hadoop 2.0
Hope this clarifies.