Role Playing Game: A game genre where the player takes on the role of the main character or guides a party of characters through a storyline in a fictional setting.
Seems pretty straight forward. However, it seems to me that you could make that definition work for a lot of video games that you wouldn't normally categorize as RPGs. So what makes an RPG an RPG then? In my opinion, it's the fictional setting. When I think of all the RPGs I’ve played over the years, most of them take place in a fantasy setting (thank you Mr. Tolkien) with the remainder falling pretty squarely in the realm of science fiction. All of that having been said, there are exceptions to the above. Cartoons, comic books, post-apocalyptic, and horror are some of the other settings you might find in an RPG. And there are probably more.
So far it seems like pinning down a game genre is some slippery business, right? Well, guess what? It gets even trickier when you add in sub genres, which is what we are about to do.
JRPG: Japanese Role Playing Game
This style of game is not always made in Japan. South Korea, China, and even European countries have been known to produce this type of RPG. The defining characteristic of a JRPG is that rather than there being only one character the player controls, there is a party of characters. Often times members of the party will leave or be substituted for another as the plot of the game unfolds. This style differs from most Western RPGs in that it uses a team dynamic rather than the lone protagonist approach.
Examples: The Final Fantasy series, Divinity: Original Sin
ARPG: Action Role Playing Game
This type of RPG is all about the hack and slash. Your decisions don't really effect the plot of the story. (Unless you decide to stop playing altogether.) All of the character development tends to happen in the scripted story line. The only things you have to do is kill all the bad guys, although, looting and assigning your skill or talent points might help you with all that killing....
Examples: The Diablo series, Marvel Heroes 2015
The Sandbox RPG genre gets its name from the concept of a, well, sandbox. The idea being you have a large area to play in with just a few outer boundaries and you can go and do pretty much whatever you like. This style of game typically still has a main plot line to follow, but how and when it is followed is left to the player.
Examples: The Elder Scrolls series, The Fallout series
MMORPG: Massively Multiplayer Online Role Playing Game
Okay. Take your Sandbox RPG, cross it with the hack 'n slash of your ARPG, the party of your JRPG at times, and add several hundreds to thousands of other people, and you will have a typical MMORPG, or MMO for short.
Examples: World of Warcraft (duh), Eve Online
Attribute: One of the defining characteristics of your character. Usually given as a numeric value and used to determine your effectiveness at a skill, combat or otherwise.
Class: Typically what your character does or what they are. E.G. Warrior, Healer, etc.
DPS: Damage Per Second. This can refer to how much damage a character can deal in a second or it can refer to the job of a character to deal damage within a party of players.
Experience Points: Also commonly referred to as XP. These are points that are gained by completing tasks, killing adversaries, exploring, etc. They are used to level up your character which can boost your attributes, give you talent points, new abilities, and so on.
Healer: One of any class in a game that can heal or the person whose job it is within a party to heal the other players during combat.
NPC: Non Player Character. Any character not directly controlled by the player or another player.
Party: A group of player characters working toward a common goal.
PC: Player Character. Any character controlled by the player or another player.
PVE: Player Versus Environment. When the player character is dealing with problems and/or adversaries that are controlled or generated specifically by the games programming code.
PVP: Player Versus Player. A game mode where players actively engage in combat between one another. This can be done in spaces that are specifically set up for it like arenas or out in the “open world” if the game has been coded for it.
Race: The type of people you come from. E.G. humans, elves, dwarves, etc.
Tank: One of any class in a game whose job it is to take the brunt of the attacks from enemies so that the other members of the player party don't have to, thus making the job of healing easier as the healer mainly has to focus on healing the tank.
The genres and examples above are the most common and popular in the RPG genre. There are some other sub-genres, though, as well as many more examples of games within those sub-genres. Many of them are worth getting to know personally. I humbly suggest you go out and try some of them.
Regina and Rhonda did a podcast discussing the basics of RPGs. You can find it here.