Runescape, according to the Guiness Book of Records, is the largest free-to-play MMORPG in the world, with over 10 million active accounts and 156 million registered accounts.
The game is very open-ended, leaving the player with a huge choice of what to do. As with most MMORPGs there are quests to complete, NPCs to battle, treasure to find, trade and sell, and a large number of minigames 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 levelling their character up. There are 26 skills currently in the game (with a new one added roughly every 12 months) varying from combat related (attack, defence, range) to 'skill' related (woodcutting, mining, crafting, fletching). As a player uses a skill (chopping down a tree, for example) then they will gain experience in that particular skill. The higher a level a player has in a particular skill, the more he is able to do within that skill (cut 'better' trees in woodcutting); it also will unlock content, such as new areas to explore or quests to do.