Fare Class (or Fare Code) is the letter you see in your ticket reservation; the common ones are:
F- First class
J- Business Class
Y- Economy Class
On British Airways however, I recently had a ticket with the fare code of
I, what is this class?
Club Europe/World (Discounted). (A Business Class 'flavour'):
I is a discounted business class booking code. Different airlines use the same booking codes for different purposes, but in general J is full-fare business, D is lightly-discounted business, and I is deeply-discounted business. Likewise, F is in general full-fare first class, while P and A are discount first-class. Coach has many more discount codes.