Copying from Wikipedia:

In Greece, in particular on the island of Crete, Tsikoudia or Raki is a grape-based spirit made from the distillation of pomace, i.e., the pieces of grapes (including the stems and seeds) that were pressed for the winemaking process. It’s a pomace brandy comparable to an Italian grappa or French marc.

If you ever visit Crete, I’m sure you had the opportunity to drink Tsikoudia. A lot of people believe that Tsikoudia is a very strong spirit. This is partial true. In fact contains as much alcohol as Whiskey of Vodka. The real true is it has a very strong taste. This is the reason many people believe it as a strong spirit. The main feeling of drinking Tsikoudia, is a “burn” at the mouth and/or throat. A strong feeling of this burn is the main symptom to understand you’re drinking it the wrong way!

The correct way (according my “strong experience”) is: “Swallow it, don’t drink it” and eat something cool afterwards, like a raw vegetable (cucumber, tomato etc) or a fresh fruit (apple, watermelon etc). This is how the old Cretan people used to drink the Crete’s tranditional spirit, learned by “the original source”