I think the Wikipedia explanation covers it pretty well:
A role-playing game (RPG and sometimes roleplaying game) is a game in
which players assume the roles of characters in a fictional setting.
Players take responsibility for acting out these roles within a
narrative, either through literal acting or through a process of
structured decision-making or character development. Actions taken
within many games succeed or fail according to a formal system of
rules and guidelines.
There are several forms of RPG. The original form, sometimes called
the tabletop RPG, is conducted through discussion, whereas in live
action role-playing games (LARP) players physically perform their
characters' actions. In both of these forms, an arranger called a game
master (GM) usually decides on the rules and setting to be used and
acts as referee, while each of the other players plays the role of a
Several varieties of RPG also exist in electronic media, such as
multi-player text-based MUDs and their graphics-based successors,
massively multiplayer online role-playing games (MMORPGs).
Role-playing games also include single-player offline role-playing
video games in which players control a character or team who undertake
quests, and may include capabilities that advance using statistical
mechanics. These games often share settings and rules with tabletop
RPGs, but emphasize character advancement more than collaborative
Despite this variety of forms, some game forms such as trading card
games and wargames that are related to role-playing games may not be
included. Role-playing activity may sometimes be present in such
games, but it is not the primary focus. The term is also sometimes
used to describe roleplay simulation games and exercises used in
teaching, training, and academic research.
Or in short form: any game where the principal focus is on roleplaying is an RPG.
So yeah, a lot of stuff is an RPG. Video games aren't covered here because they have their own SE site already. Board games & card games have their own SE. Anything else is largely fair game.
You don't see a lot of questions about things like LARPs here despite them being on-topic, just by their nature. Each LARP has its own rules & community, and they're so small that they're insular. People just ask each other, rather than asking a wider Internet audience.