I’ve seen a good number of arguments between game fans about what is and isn’t an RPG, but the actual answer might be a bit disappointing. They want to be able to point at a game in an argument and definitively say "That's not a [insert genre here]!" And they argue about this constantly, when what they are really saying is "I don't like [insert game here], so I am going to say it is not [insert genre here] in order to make my dislike seem more legitimate." While a lot of hardcore (and argumentative) players want genres to have a hard and clear set of criteria, the reality is that they aren’t. To understand this, we should go back to where they originally came from. The answer lies in two words: Shelf Space.

In the beginning, long before any kind of digital distribution, video games were sold from wholesalers to retailers and put on the shelves in brick and mortar stores. No matter what you're selling, be it video games, kitty litter, or shampoo, the retailer will want to put similar things together. That way, the customer will intuitively grasp the similarity and be more likely to buy the products he or she wants. This helps the customers find things, as well as suggests other products to the customer that he or she might like. So this is what began the idea of video game genres. They are roughly categorized onto where they would be sold by a retailer, and they are done pretty haphazardly by employees quickly.

Video games tend to have far too much bleed-over to ever have any real hard classification system. It'd be way too easy to find examples of games that defy any set of established rules you have, which is why most of us on the development side don't even bother. Instead, we often use a general and simple set of rules to determine whether a game is of a specific genre. This doesn't preclude a game being part of multiple genres either. We generally leave it up to the retailers to do the final call when they are putting games on shelves. So here are the (rough) general guidelines:

• Does it involve shooting enemies or a first-person perspective? Shooter.
• Does it involve playing a real-world sport of some kind? Sports game.
• Does it have experience points and levels? RPG
• Is it online-only? MMORPG/Online game.
• Is there manipulation of blocks involved? Puzzle.
• Top-down view with lots of little selectable things? Strategy.
• Race tracks and vehicles? Racing.
• Skybox in first person? Flight sim.
• Plastic instrument? Music.
• One person on the left fighting another on the right, with health bars up top? Fighting. 
• An 80s classic that old people would recognize (e.g. Pac-man, Centipede, Galaga, etc.), or a game with 8-bit aesthetic? Arcade. 
• Didn't fit? Action/Adventure.

And that's really it. Can games get more than one "yes"? Sure. Most of the time, nobody on the development side really cares. If you want to say your game is an RPG, go ahead. If you want to say it has "RPG Elements", go ahead. We rarely bother trying to classify things because we know that there are plenty of games that break any hard set of genre rules - and as more games come out, it will continue to do so.