Trade Unions or Craft Unions. These labor organizations restrict membership to the practitioners of a specific craft or trade (shoemakers, electricians, plumbers and so forth). This type of union organization has its roots in the guild system of the Middle Ages and has been present in America from colonial times.
Trade unions are often cited as examples of horizontal organization, meaning they seek to include the entire population of similarly skilled workers.
Industrial Unions. These labor organizations seek to include within their ranks all workers, skilled and unskilled, who are employed within a given industry (railroads, steel, etc.). The Knights of Labor and the Industrial Workers of the World were examples of this form of organization.
Industrial unions are often cited as examples of vertical organization, meaning they seek to include the broad population of workers within an industry, skilled or unskilled.
The terms horizontal and vertical are also used to describe forms of industrial organization.