The difference is that farm coins are a lot easier to get then farm cash.
To get coins you could either collect from trees, crops, or animals, help a neighbor out, sell items, buy them, (with real money) and collect bonuses from other facebook friends.
To get cash you could either buy them (with real money), or gain a level. (each level rewards you with 1 farm cash. its unfair, but one of the onlys ways to get it.
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