[Llarry]

In the era before ship-launched surface-to-air-missiles, the most effective ship defense against aircraft was the gun. U.S. Navy World War II warships (and auxiliaries, too) bristled with anti-aircraft armament from .50 caliber to 20mm, 40mm and dual-purpose (either AA or anti-surface) 5-inch 38 caliber guns.

Wartime and postwar studies of the effectiveness of AA armament concluded that smaller guns were of limited effectiveness against a determined attack (notably, the late WWII Japanese suicide attack). The most effective guns were the 5-inchers. To some extent, the smaller guns (20mm particularly) were deemed primarily to be a morale factor, even though the chances of them stopping an attack were minimal. See the first photo for a picture of a twin 5-inch gun mount.

Before the war, the U.S. Navy had authorized a class of light cruiser armed with 5-inch main guns, but lots of them. The USS Atlanta (CL 49) was the class leader. She had eight twin 5-inch gun mounts -- later ships had only six due to stability concerns. The original intended use, conceived well before WWII experience with AA guns, was that of destroyer flotilla leaders or flagships. But soon the Navy found that the 5-inch-armed CLs were quite useful in air defense. Note that in the U.S. Navy, CLs were generally armed with 6-inch guns as their main battery.

The Atlanta class, which started joining the fleet late in 1941, had marginal stability. Later ships omitted two gun mounts to help with that issue. An improved successor class was built, but the lead ship was the USS Juneau (CL 119), which was commissioned in 1946. In 1949, these 5-inch-gunned cruisers were redesignated anti-aircraft cruisers (CLAA) but of course it would not be long before missiles would be the weapon of choice.

So a summary of U.S. 1940s USN cruisers would look like this:
CB - Large cruisers (sometimes referred to as battle cruisers), only several built
CA - Heavy cruisers armed with 8-inch main battery guns
CL - Small cruisers (often referred to as light cruisers) armed with 6-inch main battery guns
(All the above types also were armed with 5-inch secondary guns)
CLAA - Antiaircraft cruisers armed with 5-inch guns.