Some of them seem pretty much in line with what you'd expect; lightweight, medium weight, heavy weight. It was introduced as a class between light heavy and heavyweight. Heavyweight has historically not had a weight maximum while light heavy has generally been set at around 180 (178 under current rules). Up until the late 70's cruiserweight was used merely as a catch weight because most heavyweight champions such as Joe Louis or Rocky Marciano were under 200 pounds, but as boxers got bigger the need for a new weight class became evident.