Apache Calcite - Understanding RelNode vs RexNode

[M Singh]

Hi:
I am trying to understand the difference b/w RelNode and RexNode.  I have taken 
a looks at the javadocs and a few examples, but am still not clear what role 
they play.
It's a newbie questions, but if there is any good blog/documentation/article, 
please let me know.
Thanks
Mans