Many people when seeing a harvestman assume it is a spider, but though it is an arachnid it's not a spider at all. It has eight legs, but the body is all one big blob, at least visually, whereas spiders have a clear cephalohorax (head/body, to which the legs are attached) and abdomen (usually bulbous lump attached, beyond a narrow waist of sorts). Harvestman also lack venom or silk glands and have only two very poor eyes compared to most spiders eight higher-fidelity eyes. Wikipedia has the full details.
The picture above shows one I found in the house, exhibiting the long gangly legs (one missing) and also a close-up of the body. This page has some much better photographs, albeit of more exotic foreign versions.
Note that some people (Americans in particular) call this creature a daddy longlegs, whereas in the UK that name tends to refer to the similarly gangly Crane fly.