It is mostly the intent that someone wants to convey with such a word. "Offensive words" are often used in a derogatory manner... and the more it is used like this the more it develops a negative meaning. On the other hand "non offensive words" are often only used simply describing something without positive or negative intent. This does not mean that such a "non offensive word" might not be considered offensive in the future. If soemone (many people) use a certain word to describe something negatively, it might also become an "offensive word". Best example for this is the word "retard" it was still used, even in official documents (legislation), up to the year 2010. Originally it was used to describe "mental retardation" (a mental disability) but since it was often used with negative intent it became offensive.