Most homebrewers are aware of the general difference between hops, but what do these really mean and how can you apply that knowledge for choosing the right hops for an ale.
The three most important factors to how a hop will affect your beer is the alpha acids, beta acids and essential oils.
Alpha acids are the main cause of bittering in hops both soft and harsh. An alpha acid called Humulone adds a soft bitterness while Cohumulone adds the harsh bitterness. There are other alpha acids that have an effect but it is a mix of these two that are seen to be the biggest components and are the focus on what alpha acids controlled in a hop. Beta acids also add bitterness but to a lesser extent.
Both alpha and beta hops take some time to be broken down in your homebrew, beta acids especially. This is why bittering hops are placed in the boil first, they need the longest exposure so that the acids are both brought out and broken down to add the bitterness you want. This is also why later hops are not seen as important to bittering because they lack time to contribute all their acids.
So for a less bitter beer you want less alpha and beta acids, for a softer bitterness you need one with higher Humulone and for harsher one with higher Cohumulone.
The vital part of aroma hops the essential oils they add to your brew. Hops produce many oils but only around 20 are important for the aroma and flavouring of your beer. Being what creates the aroma in your beer they are also what contributes to the smell of hops, Myrcene gives you that fresh hop smell while Humulene and Caryophyllene give general hoppy aroma. These three oils are what make up the majority of oils in your hops and thus have the biggest contribution to aroma and flavour.
So for less aromatic beers you want a lower amount of essential oils.
One issue with choosing your hops though is even in a variety there is huge variation between plants. For example the amount of oil in a batch of Cascade hops can vary from 0.28 to 1.78 ml/100g, that is a huge variation between your hops. Essential oils are mainly where you see huge variations but alpha and beta acids also can vary quite a bit. This is down to where they grew and how they grew the second being an incredibly hard to control factor. Especially from a hobbyist perspective you have to accept that you will not have complete control over your hops.
Don’t let that dishearten you though, you can still make smart decisions with hops to craft the beer you want to make.