Manufacturing methods can essentially be divided into two categories: additive and subtractive. Subtractive methods include any process in which material is removed to product the desired form (e.g. milling, craving, cutting), whereas additive processes (e.g. 3D printing) involve adding material. Knowing which methods to use for the task at hand is a vital part of good design. The decision of whether to use additive or subtractive manufacturing is influenced by a number of factors including materials, part quantity, and part geometry. For tips on how to choose between additive and subtractive methods in your own work check out this great article.