The first thing to understand is how hops are utilized during the boil. In short, the longer the hops boil, the more bitterness they will produce and the less flavor and aroma they will produce. There are typically three schedule parameters:
Bittering addition- typically added when 60 or more minutes are left on the boil
Flavor addition- 15-30 minutes left on the boil
Aroma addition- 0-10 minutes left on the boil
The bulk of IBUs almost always come from the bittering addition. The flavor addition is primarily concerned with flavor and the IBUs contributed are merely consequential. The aroma addition is concerned with adding hop aroma and IBUs are nil.
So, to sum this up:
Bittering addition- primary concern = IBUs (but some flavor does carry through)
Flavor addition- primary concern = hop flavor, but IBUs added are significant and should be considered
Aroma addition- primary concern is hop aroma, but flavor can be contributed depending on when it's added.
Typically, the bulk of hops is added for the bittering addition, with a proportion less for flavor, and usually even less for aroma, but none of this is black and white. A good place to start is to understand what you want from the beer hop-wise. Style guidelines can be helpful here. Each style of beer has typical hop characteristics which indirectly help determine the hops quantity and schedule.
Some beer styles can have more hop additions than listed above (e.g., IPA). Others can have only the bittering addition (e.g., Helles). A good read for this would be Ray Daniels' Designing Great Beers. This can help you understand the proportion of hops for various styles.