LOL. If there is anybody who wants to explain that short answer above...
if you need that explained you need to listen to more music and play more games and in general think more
but basically how it works is this: ambient music is good for setting atmospheric/environmental moods, and melodic music is good for character themes, main themes, final boss themes, and other things for which memorability or recognizably of a track is required
generally games with good soundtracks use both types of music; they use ambient music for situations like dark caves, forest areas, etc., and melodic music for things like character themes, etc. -- the balance tends to vary from soundtrack to soundtrack, but most good soundtracks have both types of music
example soundtracks with a good balance between the two are:
- chrono cross
http://www.youtube.com/playlist?list=PL04ADB121151A2C5E&feature=plcp- xenogears
http://www.youtube.com/playlist?list=PLC537D1513657B4E2&feature=plcp