Sea spiders are marine arthropods, class Pycnogonida, order Pantopoda, type genus Pycnogonum; as such, they are also called pycnogonids. They look like a cross between spiders and crabs, although there is wide variation, with over 1,300 species worldwide.
Pycnogonids have from 8 to 12 legs, and may have a range of other appendages (ovigers, chelicerae, etc.) that could be mistaken for legs. They often have narrow, stick-like bodies, not much thicker than their legs. Their bodies consist of three segments: a head, cephalothorax, and small abdomen. They range from 1mm to 70cm, and the buoyant effect of living undersea allows for some very long and/or twisty legs; some look like daddy longlegs, others look like an artist got carried away in designing an Especially Evil spider.
They have some interesting internal anatomy as well; their bodies are often too small to hold all their organs, and so their digestive system includes tubes extending down inside their legs; they do not need a respiratory system, as they are thin enough that gasses can diffuse throughout their body without. They do have a heart, but circulation is aided by peristalsis of the gut in the legs.
They are noteworthy as an oceanic parasite; many species have larvae, looking something like microscopic fleas, that live parasitically in shellfish, polyps, or polychaetes. Other species' young attach themselves to their father's legs. The adults are primarily grazing parasites and scavengers, feeding on almost anything soft and not too mobile.
If you want to see a sea spider, most species live in the polar regions, although they can be found worldwide. They prefer comparatively shallow water, including estuaries. They stick to the bottom, and often hide under rocks or in plants.