Spark RDD and Mahout DRM

I am continuously reading about Mahout, Hadoop, Spark and Scala; willing
to be able to add value to them. However, I am confused with 2 things:
Spark RDD and Mahout DRM.
I do know that spark’s RDD is used while working with Mahout. However, I
came across some Scala code which is using Mahout DRM or wrapping RDD to
DRM.

Thus, could anyone clarify the difference between them?

Thanks in advance
Regards