Why are there two separate packages map-reduce package in Apache’s hadoop package tree:
Why are they separated out? Is there documentation that clarifies this?
They are separated out because both of these packages represent 2 different APIs.
org.apache.hadoop.mapred is the older API and
org.apache.hadoop.mapreduce is the new one. And it was done to allow programmers write MapReduce jobs in a more convenient, easier and sophisticated fashion. You might find this presentation useful, which talks about the differences in detail.