Well, here is my explanation of an RPG!
SiphonicSugar Okay, do you know what RPG stands for? 6:08:17 PM
SiphonicSugar Well basically, RPGs have battle engines, which are run when ever a battle needs to be fought, like if there is a boss battle or a random battle when you are traveling in the game. 6:10:25 PM
SiphonicSugar You progress through the world with a story line and you are supposed to complete some final objective, or something. 6:11:04 PM
SiphonicSugar And of course there are NPCs standing around and stuff that you can do side quests for and buy items from. 6:11:51 PM
SiphonicSugar Along the way to distract you. 6:12:03 PM
SiphonicSugar You can have conversations with them too. 6:12:35 PM
SiphonicSugar Do you know what the Elder Scrolls V: Skyrim is? 6:13:09 PM
SiphonicSugar Well if you do, that is an example of a 3D RPG. 6:13:22 PM
SiphonicSugar You are a character that plays a roll in the world to do something. 6:13:43 PM
SiphonicSugar And that is my explanation.
It would help if you also specified what RPG actually stands for, if you intend to explain what is one. In this case, Role-playing game. :P
That said, nowadays there are two types of RPGs: western RPGs (Skyrim, Dragon Age, Risen, Diablo) and JRPGs (Final Fantasy, Dragon Quest, Tales of Xillia/Phantasia, Xenoblade Chronicles). The western genre usually have a gritty, serious and depressing atmosphere, while the Japanese genre is usually more humoristic, cliché, faster-paced and somewhat happy atmosphere.
Also a lot of RPGs have weapon/character leveling up elements, but I don't know if that's required. Basically, you can personalize your chatacter and explore more. Some people went as far as considering Metroid, Simon's Quest and Zelda RPGs.
Lol. I forgot to specify what RPG actually stands for. :P
Here's (https://www.wikiwand.com/en/Role-playing_game) a really in-depth article on all types of RPG's. It seems that the only RPG's that are focused on these days are the video game ones(I ain't complaining tho).
Also, all games have some RPG in them, no matter how little.
Yeah, back in the 70's, RPGs were mostly dice and pen games, then eventually evolved into board games and video games. Nowadays, western RPGs have been slowly replacing JRPGs in the video game world. Even JRPGs have been integrating more and more western RPG elements since the past few years (Xenoblade Chronicles come to mind)