Answer:
An RPG (or Role Playing Game) is played, as the name suggests, from the role of a hero who moves through a world manufactured to interact with his/her every decision and change accordingly. This requires a fairly large amount of text, which in turn creates a story for you to move through.
Making an RPG takes several months or even years of painstaking preparation, design and research. However the basics required are
A) A compelling story told in several ways to allow for the player to manipulate it to their will through their choices. For instance, killing the Dragon in chapter one may allow the player to open a chest in chapter 3, but sparing said dragon will allow player to summon it in battle at the end of the game
B) A very good understanding of Mathematics. This is because every piece of armour, weapon, landscape and spell is a new variable. Weapon A held by level 10 knight on high ground against Armour B worn by level 5 thief on low ground protected by a spell to decrease damage... every swing of the sword will require a new sum as HP changes as well.
C) An understanding of programming - is it a text based RPG? Or maybe a turn based battle such as Final Fantasy Tactics. Which software is best to code it in? Why aren't the monsters (drawn months ago and painstakingly encoded into 3D) reacting?
Sorry if this answer was long winded, but your question was more complex than you probably thought.
If you wish to see how it is REALLY done some developers (Core/Eidos in Derby, makers of Tomb Raider for instance) sometimes allow a tour of the studio to people seriously considering a career in "Making Games" that will show you just how much time/money/effort/etc goes into making a game