(Newbie) Merging two data frames

This one is an easy question. I am looking for the "idiomatic" way to do it.

I have two large data frames. I want to "merge" them. What is the
idiomatic way to say "match the rows from dataframe 1 to the rows in
dataframe2 which have the following fields the same: Identifier, Year
and Quarter? (These three fields form something like a composite
primary key in SQL). Then tell me which rows you could not find a
match for.... etc.

-- Vivek Satsangi
Student, Rochester, NY USA