Tuning a game involves a series of activities, which are related to balancing, but with a distinctive aim. Tuning has to do with making a game fun to play from beginning to its end, possibly for players at the most different skill levels (though it is true that few games specially aim towards very skilled players, sometimes).
With this last subject we enter a very dark area of games design in general and videogame design in particular because we get to the point of defining what is fun. Unfortunately, a definition of fun would require an entire book for itself, so we suggest you to refer to this very interesting Gamasutra article to begin your research into the topic of fun in Videogames found at http://www.gamasutra.com/view/feature/173545/fun_is_boring.php.
Sid Meyer, the founder of MicroProse and designer of several popular strategy games like the Civilization series (http://en.wikipedia.org/wiki/Sid_Meyer) once said that games can be described as a series of interesting choices...