Although salmiakki resembles black licorice in color and often complements it in flavor, it is actually the salt ammonium chloride. Synthesis of ammonium chloride is a common experiment in Finnish chemistry classes.
Salmiakki candies are almost always black. The only other colors used are white and variants of grey. Since pure ammonium chloride is a white powder, the reason why black is the preferred colour for salmiakki is somewhat unclear. It might be partly to provide a "tougher image" for the candies, and partly because of the liquorice used to vary and soften the salty flavor.
The canonical shape for salmiakki candies is a four-cornered lozenge. This shape is so popular that in Finnish, the word "salmiakki" can sometimes refer to the lozenge shape, instead of the candy or the salt it is produced with. This is particularly common in computer science when discussing the Unified Modeling Language. Round salmiakki candies are also very common, either disc-shaped or complete spheres.
In addition to being used in candy, salmiakki is also used to flavor vodka and distilled rye brandy.
Salmiakki is extremely popular in all five of the Nordic countries, but is lesser known elsewhere in the world. Most non-Nordic Europeans have heard of salmiakki, and sometimes tried it, but are generally not especially fond of it. A notable exception is the Netherlands, where salmiak (salmiakki) is very popular. Because of this, the Netherlands have been called "the sixth Nordic country".
Salmiakki is virtually unknown outside Europe. Some Finnish ice hockey players playing in USA leagues have tried to import salmiakki among their American teammates, but so far, results have been pretty much hopeless.