Free to play games offer users the option to play without any initial fee but provide optional purchases or other payment options. Free to play should not be confused with freeware, which usually refers to software that is completely free.
Some free to play games feature in-game advertising, and others allow users to pay for exclusive items, premium content, or upgrades to "full" versions of the game. An example of this is TrackMania United Forever, which contains all the content of the free game TrackMania Nations Forever plus additional environments and cars. The vast majority of Free to Play games use Real Money Transactions to fund their games.
With the inclusion of in-game purchases in the iTunes App Store, many mobile game developers decided to release games that are free to download, but later give an option to expand or upgrade the game experience through purchasing additional content. These games have come to be known as "freemium" content. A good example is the MMORPG, Pocket Legends, which allows you to install and play the game for free up to level 12. This includes several dungeons, as well as item pickups. However, to increase in level, attain additional items to customize your character, or visit new maps, you have to pay for the content via in-game purchasing.
The term "freemium" was coined in 2006 by a reader of capitalist Fred Wilson's blog. However, the term has gained much more common use with the massive amount of free-to-play content on mobile devices in recent years.