There are two basic ways to interact with calculators, each have multiple names depending
on where you're from. The way "most" calculators work is called "algebraic" or "operator infix"
notation. The basic way to describe this is how to calculate 1+1. On an algebraic calculator,
such as the iPhones built in calculator, you calculate this like so:
In RPN, a slightly different method is what you do:
In other words, you add items to the stack (using the "R" key), and then add them.
To some this seems backwards, but many people find they can manipulate more data intuitively with
the RPN system. The reason for this is that since each operation explicitly works on either the
bottom or the bottom two items in the stack, all operators behave the same. There is no hierarchy,
and no way to confuse 1+(3*2) with (1+3)*2, in RPN, it is simply "2R3*1+"
or "1R3R2*+", respectively.
There is a lot more information freely available about RPN and Algebraic Notation.