Any recipe for genuine, traditional-style absinthe always includes three distinct groups of herbs: the primary "holy trinity" herbs, taste-enhancing herbs and, finally, herbs added for aroma and colour.
The herbs that essentially define absinthe as a drink, or set it apart and that largely give absinthe its characteristic taste and cause its unusual effects. These three herbs are always present, in any absinthe made:
Wormwood
It is bitter in taste and contains an oily substance called thujone – and, of course, it is thujone that is responsible for the unusual absinthe effects.
Anise
This gives absinthe its typical liquorice flavour and scent. Its oil also plays a part in creating the unusual
louche (clouding) effect when water is added to absinthe.
The seed (correctly the fruit) of this flowering plant is sweet in taste and very aromatic.
Fennel
The liquorice flavour from Fennel is a little sweeter than anise's but certainly much less intense.
Fennel balances the bitterness of wormwood and tones down the sharp edges of anise.
*Source of information
http://www.absolutelyabsinthe.com/absinthe/herbs.html