It's got long legs  - but is it a Spider or a Harvestman? Here's how to tell...

Most Arachnids (invertebrates with jointed limbs) have undivided bodies, e.g. mites, scorpions, ticks, and harvestmen. 
Spiders are different, their bodies are divided into two parts, a prosoma (cephalothorax) and an opisthosoma (abdomen), joined by a bridge - the pedicel.