We grew up knowing and loving them as daddy longlegs. My mother taught me to be unafraid of them, and of moss. I remember her on that morning, by a path at our little camp on Oneida Lake, so clearly. It is one of my first memories.
They are not technically spiders though they share the same Class, Arachnida, along with true spiders, scorpions, and tics and mites.
They have their own Order, Opiliones,
They are mistaken in appearance for cellar spiders, Pholcidae, which are also harmless. These others are called Daddy-longlegs Spiders, are true spiders, and have two main body parts, a cephelothorax and an abdomen, while Opiliones have a pill-shaped body. Pholcidae can make going into the cellar during the summer somewhat unpleasant.
Arachnologists are using the common name harvestmen for daddy longlegs. Not me.
They are most definitely not poisonous and any view to the contrary is myth.