For those of you who don’t know it, Steam is a PC game store that is considered to be the “iTunes store” of gaming for the PC and the Mac. As of today, Valve(the company behind steam) allows companies to publish their games using the free to play model. Essentially, some games will be free to download and play. However, there will be some games of which will have some add-ons in the game which you can get if you are willing to spend some money.
Farmville on Facebook(a social network) is an example of this. The game itself is free to play, but there is an in-game currency called Farm Cash. You can get more of this currency if you are willing to spend money. However, you cannot change farm cash back into real money.
If you have a Steam account, then I suggest that you check it out right now and let me know what you think of it by emailing me on Adi@adiman423.co.uk