Most people are so much worried about quantity of Hops to be used that they forget that at what stage a hop is introduced as as important as the right quantity.
Bitterness hops are often introduced early in the boil (at least 30 min before flame off). The high temperature boil will allow for the resins to be extracted to get the maximum bitterness. However the downside is that it will significantly hamper your ability to retain the aromas.
Last 10-15 min before the beginning of the wort cool is the best time to introduce aromatic hops. If you can cover the wort as well, it will allow you to trap the aromas that most homebrewers simply die for. It allows for an optimal mix of extraction of resins (bittering oils) and aroma to be infused in your wort.
IPAs and other hoppy style will call for Dry Hopping. It is essentially infusing hops (preferably in the leaf or cone form rather than pellets which disintegrates into powder and hampers clarity)
Since there is no heating, the bitterness is low but the hoppy aromas are fully extracted in the presence of the alcohol which acts as a solvent. I typically infuse hops 2 weeks prior to bottling which allows for maximum extraction (hops are expensive spices) and allows all the complex flavors and aromas to blend well.
However care should be taken to put hops in a bag which is weighed down with a glass marbles. A floating leaf is a perfect harbor for unwanted microbes (read mold) which is just waiting to make your precious into barley cider.
Brewmasters also use dry hopping to get consistent flavors & blends in their bottles and remove the differences between different batches. However care needs to be taken as hops can easily overpower your beer.