Runescape, according to the Guinness Book of Records, is the largest free-to-play MMORPG in the world, with over 10 million active accounts and 156 million registered accounts.
It is played through your computer's web browser, there is nothing to download, but it can take a long time to initially load for play (every time you log-in to play).
Runescape is an open-ended Massively Multiplayer Online Role-Playing Game (MMORPG); it is entirely up to the player to decide what to do, when to do it, and how to go about it. Runescape has many quests to complete, NPCs/monsters to battle, treasures to find, specific quest/reward locations to explore/complete as a part of their extensive game tutorials and game-play introductions, a 'live' market system for buying and selling items/goods, and a large number of mini-games to play. There is also player-vs-player combat, but this is entirely optional and avoidable if so desired.
The main focus for most players is to level-up their characters. Leveling your character is accomplished by raising skills. There are over 20 skills in the game; varying from combat related (melee, ranged, magic) to 'skill' related (woodcutting, firemaking, mining, smithing, crafting, fishing, cooking, runecrafting, magic, etc). As a player uses a skill (chopping down a tree, for example) they will gain experience in that particular skill. The higher a level a player has in a particular skill the more s/he is able to do with that skill (cut-down 'better' trees in woodcutting, use higher grade axes for woodcutting, etc). Raising skills can also unlock content, such as new areas to explore or quests to do.
The 'strength' and 'defense' stats are used as a sub-set of the attack skills and developing these allows for the use of higher grade weapons and armor, just as how other skills develop.
Runescape has extensive, and on-going, free game-play. People who play for free are limited to a restricted area within the game's world, but this area is very extensive; encompassing over a half-dozen major cities/towns, many dungeons and areas for exploration, a large massive multi-player game arena, and much more. 'Members' have access to the whole world, all the quests, all the skills, all the spells/prayers, more slots in their 'bank account' for storing items, all the higher grade materials for weapons, armor and equipment, etc. But free players have access to a great deal of what Runescape has to offer, and many players remain 'free' and continue playing without purchasing a membership. Jagex has made a solid effort to maintain and develop content for the free-play experience as well as to continue providing more for their paying members.
Jagex also regularly hosts special in-game Runescape events around holidays. These holiday events are usually mini-games and quests based around on-going holiday themed stories and characters that they have developed and created. These special holiday events also gift special limited-time holiday items and emotes (an 'emote' in Runescape is a special animated action that your character can perform - many of which need to be unlocked by completing quests/events).
Runescape has a great deal of good-spirited humor built into the game world, much of which is 'tongue-in-cheek'. It is pervasive throughout the dialogue, quest story-lines, NPC characters, etc. Following the story-lines and quests (and actually reading the dialogue) can be humorous as there are often over-arching consistencies that Jagex has maintained throughout Runescape.