For the ones of you that don't know RPG stands for Role Playing Game, and is one of the most played type of game nowadays.
You are the main hero, and you interact with other characters also called NPC-s ( or Non Playable Characters if you're playing single player). They will give you quests to do, and you have to do them, in order to get experience and advance to higher levels.
The story has the main quest, that will end the game when finished, and usually a lot of side quests, that will help you develop your character. The side quests are not necessary, but will get you deeper into the story and sometimes that is really worth it!
Most RPG games allow you to choose your type of character at the beginning. Usually there are several types of characters, all with different attributes, but there are three main categories to choose from: wizard, fighter and archer. These will take different names and attributes and will be further more differentiated into subcategories, depending on the game. For example, the wizard can become specialized on different categories of spells, like earth, water, dark magic, white magic, fire, lightning, nature.
How do you grow your character? Well this depends from game to game, but basically you have :
-life, called life points in many games representing your health
-mana, or mana points representing the wizardry point you have left( these point allow you to do spells, if you don't have them you won't be able to cast spells)
-stamina, also found by other names, depending on the game, this represents how much time you can run, of do special moves.
Besides these three there are a few other primary attributes like:
-strength - representing the strength of your character, you'll have to put points here if your character is a fighter.
-dexterity -representing the dexterity of your character, usually important for archers
- intelligence - representing the intelligence of your character, usually important for wizards.
There may appear some more primary attributes depending on the game but don't worry they are usually explained!
Experience - this is heart of the game, and this (along with the story) will keep you in front of the computer for days! Basically, when you kill monsters you get experience, you also get experience when you do the quests.