Most of the other replies here say it’s all about size, and size does matter, but it’s not the whole story. Generally, cruisers are designed with capabilities

Most of the other replies here say it’s all about size, and size does matter, but it’s not the whole story. Generally, cruisers are designed with capabilities that allow them to operate anywhere on the ocean on their own, or as the lead vessels in small groups of ships (along with frigates and destroyers). Destroyers are usually smaller and have a more limited set of capabilities, and are intended to operate as ocean-going support ships in a combat group. Frigates are usually even smaller, with similar set of capabilities to a destroyer, but more limited equipment and intended to operate relatively close to their home ports. Every navy has different sizes for each, with the USA having some of the largest ships of each designation. You should also keep in mind that each type of ship has gotten progressively larger over time; as of WWII, destroyers were roughly 2000 tons, and cruisers were about 8000 tons.