Prior to WWII, engine power was so scarce that there were several types of bombers, light, medium and heavy, all tuned to a particular performance niche. As fighters grew to be able to carry the same sorts of loads at even greater speeds, light bombers were replaced around the 1950s and the term fell from use.
Light bombers of World War II were single-engine or, less commonly, twin-engine aircraft with a bomb load of about 500-1,000 kg. Designs that could be described as light bombers included the Fairey Battle.
Some of them were dive bombers. Light bombers were also the only type of bombers operating from aircraft carriers. Some twin-engine light bomber designs were also successful when converted into heavy fighters or nightfighters. eg Bristol Blenheim.