The bittering, flavor, and aroma characteristics of hops are created by a two major types of chemical compound contained in the cone-shaped hop flower: acids and oils.
A beer needs bitterness for balance, but it needs flavor and aroma to be unique
Alpha acids (humulone, adhumulone, and cohumulone), which form iso-alpha acids in the boil
Beta acids, which are the hop's essential oils (primarily humulene, myrcene, caryophyllene and farnesene).
Alpha goes bitter in the boil, Beta during fermentation.
Oils come out straight away in a boil, acids take time. This is why they are added at the start of the boil.
Cohumulone: Traditionally avoided for harness, used a lot more now
Flavour hops are added 15 minutes before end of boil. Aroma during the last 5 minutes.
More on oils at the hop yard.
Myrcene: (usually 20-65%) US over European preference. Citrus fruits. When added late in, or after, the boil, myrcene adds the intense, pungent aroma associated with American dry-hopped beers. When boiled for longer periods, it yields the characteristic citrus and pine aromas of American craft beer.
The predominate aroma contribution of this oil is a pungent floral quality, with a much smaller, but also present citrus and piney character. To the flavor is adds a orange, earthy character. Disappears fast, best for dry hopping.
Humulene: (usually 15-45%) Noble, comes out in dry hopping, if boiled for longer periods, humulene lends the finished beer an herbal or spicy character. It brings a elegant woody and herbal character to a beers aroma, but is best known for the spicy flavor it impacts. Also used as bittering hops when a very subtle spiciness is desired.
Caryophyllene: (usually 5-15%) adds a spicy, herbal character similar to humulene when boiled. Unknown when fresh
Farnesene: (usually 1%, up to 20% in Noble hops) Known to impart a woody and earthy character to beers, hops with Farnesene tend to be best used in late boil or during dry hopping.
All from Hop Union.
You can find a hop by smell you want at Hop Union too