Sailing and other methods like rowing (or rarely propulsion) is a way of traversing the waters of the world, primarily the seas but also sometimes lakes and even rivers. This is done on a variety of types of ships, some for more Mercantile purposes, some for Warfare or Piracy.
Many cultures and realms have gained a reputation for being especially good at naval matters. The Zardoban Empire is one such example, as is The Vilhi Sultanate which continues to need sailing as an entirely island-based realm. The Rubyfirthers are also talented sea-going merchants.
Sailing is required to cross from Iduria to Horiza, which makes up perhaps the most challenging journey a captain or crew can undertake due to the World Fleece.