A "daddy-longlegs" is a kind of spider.
Different people call completely different creatures by the "daddy" term. Most Americans and Canadians use the term for long-legged harvestmen, which are ground-dwelling outdoor creatures. Harvestmen are arachnids, but they are not spiders -- in the same way that butterflies are insects, but they are not beetles. Harvestmen have one body section (spiders have two), two eyes on a little bump (most spiders have eight), a segmented abdomen (unsegmented in spiders), no silk, no venom, a totally different respiratory system, and many other differences; not all have long legs. In fact, it's so confusing that the "daddy" term really doesn't mean anything, and it would be better to just forget it and say "harvestman" when you mean harvestman.