First of all, there are at least 2 spiders that are commonly referred to as a daddy long legs. These are the Pholcus Phalangioides and the harvestman.
They should not be confused (but often are) with the crane fly, which is also commonly called the daddy long legs, and which is probably what most people are referring to and recognise as that which is being referred to by others.
The crane fly is not classed as a spider. (Note to the quizmaster in my local). It is an insect and has 6 legs (another note to the quizmaster in my local).