An anise flavoured drink, Absinthe's flavours are derived from several different plants. The most famous of which is the "grand wormwood" (Artemisia absinthium).
Historically this has been a highly alcoholic spirit, between 45 and 75% ABV, and is traditionally green in colour (but can be colourless). The green colouring, and high alcohol content both contributed to its name of 'la fée verte' (the green fairy).
It is normally drunk diluted with water, sometimes poured through a sugar cube. But it's fast becoming enjoyed in other ways and in the mix.