For many people the name Daddy-Long-Legs is the nickname given to an animal. In many cases it is the crane fly, an insect with (as you might imagine) very long, thin legs. In some other parts of the world it may be a spider with long legs, and in some places it is a harvestman.
A harvestman is neither an insect nor a spider. It has eight legs and no wings so is more similar to a spider, but it has only one body part (a spider has two, and an insect three).