What is the difference between craft unions and industrial unions?

The members of a craft union all have a particular skill or occupation. Craft unions are often responsible for training their members through apprenticeships and for supplying craft workers to employers. Craft workers may work for many employers over time, but have a constant link to the union. A craft union's bargaining power depends greatly on its control over the supply of its workers.
Industrial unions consist of members who are linked by their work in a particular industry. Typically, an industrial union represents many different occupations. Membership in the union is the result of working for a particular employer in the industry. Changing employers is less common than it is among craft workers, and employees who change employers remain members of the same union only if they happen to move to other employers covered by that union. Another difference is that whereas a craft union may restrict the number of skilled craftsmen to maintain higher wages, industrial unions try to organize as many employees in as wide a range of skills as possible.