What is the difference between the old and the new MR API?

With the release of Hadoop 0.20, new MR API has been introduced (o.a.h.mapreduce package). There is not much of significant differences between the old MR API (o.a.h.mapred) and the new MR API (o.a.h.mapred) except that the new MR API allows to pull data from within the Map and Reduce tasks by calling the nextKeyValue() on the Context object passed to the Map function.

Also, some of the InputFormats have not been ported to the new MR API. So, to use the missing InputFormat either stop using the new MR API and go back to the old MR API or else extend the InputFormat as required.