The Firm
  
A firm is a business, i.e., any entity that sells something and makes a profit. It might be Walmart selling everything almost everywhere, a one-man band as an LLC, selling music on the street corner, or anything in-between. Firms are what make our goods and services travel, and keep the lights of the economy on.
What do firms have in common? In the U.S., it's an Employer Identification Number (EIN). Like a Social Security number, but for firms.
The study of the firm is the realm of microeconomics. It largely assumes firms exist to maximize profits, and explores behavior and effects surrounding this end-goal.
Nothing to do with the book, but if you haven't read it, well, you should. Or see the movie. Cruise at his best. Ish.