Dill, or Anethum graveolens, is a medium height perennial herb that displays fern-like fronds on top of hollow stalks. Dill has been used in medicine and cooking since before Roman times. Both the fronds and the seeds have culinary uses everywhere from the Middle East, Scandanavia, India and Vietnam.
Dill's distinctive aroma mostly derives from the chemical carvone, which comes in a + and a - form (technicaly, they're enantiomers, two chemicals that are mirror images of each other and non-superimposable). The + Carvone smells like dill, or caraway seed, while the - form smells of spearmint.
Dill's flavor makes it an important part of pickle recipes. It can also be used with most fish, but dominates foods quickly.
Dill should be used carefully in cocktails -- American oak barrels tend to give wine a dill aroma after the wine has aged for a few years, and the use of Dill in drinks can mimic that effect.