Free to play (often referred to by the pseudo-acronym "F2P") is term that usually means a game does not charge a subscription fee, but can mean many other things as well.
Generally with free to play MMOs there are people who play it for free, and those who pay a subscription fee on top of this. The subscription fee will usually grant players many small advantages, such as movement speed increase, leveling increase, more gold acquired and such. There is a fine line between "free to play", and "pay to win", and it is easy for "free to play" games to slip into this.
With the introduction of the in spring 2015, World of Warcraft has become borderline free to play. The reason for this is that the WoW Token can be used to add 30-days of game time to an account and can be purchased with in-game gold from the Auction House. However, since the average price of the WoW Token on US realms hovers around 100K (unlikely to go much below 95K; initially prices ranged from 30K to 65K, but steadily increased), it isn't realistic for a Starter Edition account that is level capped at level 20, even if they have activated a real account after reaching the cap. However, for returning players who have already accumulated a decent chunk of gold with inactive accounts can re-activate by purchasing a WoW Token with in-game gold and automatically use it.
Many people are thinking about whether World of Warcraft will go free to play or not, as this seems to be where the MMO market is heading right now. Many people have announced they will cancel their subscription if this goes ahead, and others have stated they will come back to WoW if it becomes free to play. The Introduction of ahas been received with strongly mixed opinions.
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