RuneScape is a browser-based (although will be available to play on iPhone and Android later in 2018) Medieval European Fantasy MMORPG by the UK-based developer Jagex Games Studio that runs on a Freemium subscription model. The free version of the game is self-contained and can be played on its own, but subscribers gain access to separate Member servers with more content in exchange for a monthly fee. The game holds the official Guinness world record for largest free MMORPG, with over 250 million registered accounts since its initial release.
Gameplay primarily involves quests, combat (both Player Versus Player and Player Versus Environment), and Level Grinding, along with a variety of side activities like Capture the Flag. RuneScape is somewhat uncommon among MMORPGs in that its quests are more elaborate than your basic 20 Bear Asses Fetch Quests, instead focusing more on plot, puzzles, problem-solving, and Boss Battles. New content, usually (but not always) members-only, is released on a weekly basis.
The game was originally released in January 2001 (and is now referred to as RuneScape Classic) in what would now be considered a very primitive form. The full world was much smaller, containing only a few small cities. There were only six quests, one server, and two developers. Classic is still playable today, although access is restricted to subscribing members who had logged in during its limited re-opening periods, and will be shut down forever in August 2018. It gradually expanded until 2004, when the whole game was overhauled completely: changes were made to the game engine, the graphics, the basic game mechanics, and the content and was released as RuneScape 2. Then, in July of 2013, Jagex released RuneScape 3, with improved audio and a new, customizable interface and kicking off the first in a series of game-wide World Events. Jagex later rereleased RuneScape 2, circa 2007, as Old School RuneScape, with its own small dev team and player polled updates.
RuneScape 3 and Old School RuneScape, colloquially referred to as RS3 and OSRS or simply Old School by players, are the two "main" versions of the game as they both receive regular updates and, thus, have their own dedicated playerbases. If you were to just say "RuneScape", you'd be assumed to be referring to one of these two versions.
It has a series of Expanded Universe novels: Betrayal at Falador (2008), Return to Canifis (2011) and Legacy of Blood (2012), all written by T.S. Church. It has also spawned three spin-off games: "Armies of Gielinor", a multiplayer Turn-Based Tactics game set during the God Wars, released through FunOrb, "Chronicle: RuneScape Legends" a Collectible Card Game, and "RuneScape Idle Adventures", an Idle Game.
It has spawned its own annual Fan Convention known as RuneFest, which has run since 2010.
A documentary about the history of RuneScape made to celebrate the game's 15th anniversary can be seen here.
Note that trope examples are assumed to refer to RuneScape 3 unless otherwise stated.