Stock: Canned, Bouillon Cubes and Powder
Commercial stock or broth products come in several categories.  Of the ones you ask about:

Canned

Bulky, but ready to use.  May be the highest quality product of the three you asked about, with the best flavor.  Especially low salt versions contain less salt than the cubed or powdered alternatives.

Cubes

Contain lots of salt.   Must be dissolved in hot water before use.  May not be as high a quality product as canned.   Requires little shelf space.  Often inexpensive on a per serving basis.  

Powder

Essentially the same as cubes, but not yet pressed into cube form.  Somewhat easier to dissolve than the cubes.

Concentrate

While you did not ask, concentrated stocks are also available on the market, which are almost like a jelly that you can add to thicken and enrich sauces, or reconstitute with water to use as a soup base.  These tend to be marketed at commercial kitchens, and may be a higher quality product than some of the others.