Spiders, mites, ticks, harvestmen and scorpions are all members of the Arachnid group. Arachnids are distinguished by having primarily eight legs (some actually have less), no wings or antennae and being made up of 2 sections - the cephalothorax (prosoma) and the abdomen (opisthosoma). Arachnids have extra appendages known as chelicerae and pedipalps. The chelicerae are for feeding and defense whereas the pediplaps are for feeding, locomotion and reproduction. These latter appendages can grow large enough to take on the appearance of legs so care must be taken when assessing your specimen.
Arachnids are primarily carnivorous and mainly intake the pre-digested bodies of a variety of insects. Spiders however eat their own silk and some Harvestmen and mites ingest solid food particles.