Who vs. Whom?

Although this may be shameful to admit as a sophomore in college, coming into COMM 160 I had no idea what the difference was between "who" and "whom".  I had never even used the word "whom" in my life and, for fear of using it incorrectly, I stuck with "who".  Thankfully, I was awarded the opportunity to learn the difference during class this past week.  As you read through a published work and stumble upon the word "who", it is easy to question whether that is correct or if it should be replaced with "whom".  The first thing to do is to replace the word "who" with both "he" and "him".  If "he" fits then the word should be left as "who", but if replacing "who" with "him" still makes sense then the "who" should be replaced with "whom".