Currency

Exchanging cash for goods and services makes it much easier for adventurers on the road.  Otherwise your barbarian's muscles might be put to use chopping wood, your thief's deft hands to washing dishes and your bard singing for his supper (well, maybe that's not such a bad idea).  So in this world of ours we need some way to pay for things without bringing along a flock of sheep or equivalent bulky trade goods.
    Originally, currencies had value due to the content of its material.  Gold, silver and bronze are the three materials we think of for use as traditional currencies.  These materials were used as coinage for three basic reasons: intrinsic value, malleability and durability.  The value of these materials (at the time of their use as currency) came mostly as a result of their rarity and beauty.  This malleability added to the value by enabling materials to be employed in a variety of decorative ways (jewelry and so on).  Malleability is also important for currency due to decreased waste in the course of production, uniformity of coinage, and the ability to print pretty pictures on it.  Durability is also important to maintain the shape and integrity of the coins.
    Historically, currencies produced by different countries (or even just different mints) were of different values for material reasons.  Coins were produced of different sizes and thicknesses, obviously using varying quantities of material in their production.  Purity of the materials was also a question.  One of the reasons the coins were stamped was as a promise regarding the composition and uniformity of the coin (whatever it may be).  
    Currency in a fantasy world does not have to be made of these specific materials, but should follow the same guidelines.  Metals tend to be the best choices for the reasons listed previously.  However, it is certainly reasonable that gold is not valued on your world, or that it is abundant (therefore less valuable).  One of the things to be careful of is in making a commonly used material into coinage.  If you use steel as currency, the value of that helmet you are wearing should be much higher than its weight in coins (otherwise you'd probably melt it down and use it for trade). 
    Representational currencies can take many forms.  The first were probably I.O.U.s or their kin.  The concept behind this form of money is that the note (or bill or what have you) can be taken to the issuer and exchanged for material of a certain value (usually precious metals, though Heinlein had some interesting suggestions on the topic).  Consequently, these currencies lose value the further from the source you get.  I probably can't give a farmer a bill from another country, though a trader or banker might find more value in it.  The same is generally true today (though today's currencies are not representational).  
    The last thing to keep in mind is that precious metals are not available everywhere.  A small country may not have mines large enough to generate as much currency as it needs.  Not every country needs its own coin (indeed, some countries today still don't have their own currency).  These countries simply utilize money coined elsewhere.  Currency of intrinsic value was freely circulated in the Medieval world.  After all, gold is gold.