I assume you're speaking of British service, since the "cruiser" tank idea was basically British, although the Soviets did experiment with a similar doctrine.

In short, a british "Light" tank was smaller, cheaper, and only armed with machine guns. A "cruiser" tank was armed with a small cannon, such as the 2-pdr, (~40mm). Cruisers gave up armor and armament in exchange for speed compared to "Infantry" or assault tanks, allowing the cruisers to exploit breakthroughs and disrupt enemy communications and supply lines, much as cavalry had once done.

British Lights were not really intended for the same cavalry role as "cruiser" tanks - they were more for reconnaissance and colonial service. They saw limited combat use in Greece and Northern Africa in 1940/1941, and were used for training later on, as well as operating with colonial groups in India, where they worked far better than they had in frontline maneuvers.