Signs can be classified in three categories: icon, index and symbol. An icon is a physical resemblance to the thing being represented (ex. a drawing of a file to indicate "files"). An index, on the other hand, is a sign that shows evidence of the thing being represented (ex. the drawing of smoke to represent fire). Finally, a symbol relies on a connection that must be culturally learned, as there is no resemblance between signifier and signified (ex. the numbers and letters).