Are you looking for Spiders, Spiderlings or Spider (lore)?
Corpse Spiders is a Primary Witch Doctor skill unlocked at level 3. The Witch Doctor tosses a small jar from which spiders swarm out, pouncing on nearby enemies. The spiders do not deal very high damage individually, but several jars can be thrown in rapid succession, and a swarm of spiders all biting at once deals considerable damage to any target.
The Witch Doctor throws a jar that breaks and releases spiders that attack nearby monsters. The skill is best spammed, with a payload of spiders delivered unto one or a group of targets.
 Skill Rune Effects
Refer to the Corpse Spiders rune effects article for thorough description of all five rune effects in this skill, including screenshots, videos, and strategy tips.
|| Corpse Spiders
Physical realm spell
|| Cost: 4.9 Mana|
Throw a jar with 4 spiders that attack nearby enemies for a total of 120% weapon damage as Physical before dying.
|| Leaping Spiders
|| Summon jumping spiders that leap up to 25 yards to reach their target and attack for a total of 144% weapon damage as Physical.
|| Spider Queen
|| Summon a spider queen that births spiderlings, dealing 630% weapon damage as Poison over 15 seconds.|
You may only have one spider queen summoned at a time.
|| Medusa Spiders
|| Summon paralyzing spiders that have a 25% chance to Slow enemies' movement by 60% with every attack.
 Skill Design
This skill was initially called Spider Totem. In the earliest version it summoned three large spiders from the earth wherever the Witch Doctor targeted the spell. They swarmed over nearby enemies, dealing poison damage.
A later iteration of the skill changed the name and function. Once it became "Corpse Spiders," the Witch Doctor summoned forth a zombie to emerge halfway from the ground. The zombie then vomited up the spiders, or sometimes they burst right out of the zombie's chest, Alien-style.
This effect was awesome, but the knee-high zombie head and torso was hard to see on a crowded field of battle, and the casting mechanism was changed to the thrown jar. Fans have suggested that the name should have changed to Jar of Spiders, but the developers preferred to keep the zombie reference, even though there's no longer anything corpse-like about the skill.
The following Witch Doctor passives provide a benefit to Corpse Spiders, runed or not:
|| Pierce the Veil
|| All of your damage is increased by 20%, but your Mana costs are increased by 30%.
The old Spider Totem Spell.
The old Carrion Spiders Spell.
Corpse Spiders was first revealed during BlizzCon 2008; it was originally known as Spider Totem (later Spider Statue) and was on the Plague Skill Tree (itself later renamed to Zombie Skill Tree). It worked like a trap (the Assassin skills from Diablo II) with a radius of poison damage. When cast a small totem would appear out of the ground, from which hundreds of small spiders would emerge, and swarm over nearby monsters, dealing poison damage.
By BlizzCon 2009, this skill had changed dramatically. Renamed to Carrion Spiders and moved to the Voodoo Skill Tree, it had a completely different visual. Flux tried it in the gameplay demo, and described it:
This skill is now called Carrion Spider, a summoning spell that causes a human corpse to erupt from the ground. This gruesome zombie sits up out of the earth and vomits out a large, milky-colored spider, before sinking back into the ground and vanishing from sight. The spider then functions as a minion, racing over to an enemy and latching onto them, dealing considerable poison damage for some duration of time. The spell can be cast repeatedly, just to watch the gruesome zombie animation, and sometimes the spider actually bursts through the zombie corpse’s chest, rather than waiting to be vomited up.
During BlizzCon 2009's Heroes and Monsters panel, the development team discussed the changes made to it. The Spider Totem had a noisy look, a player would cast this in a combat situation, and the entire screen was filled with small spiders which made it hard to concentrate. Also, they didn't like the totem thematic, as the Witch Doctor is more dark than that.
For BlizzCon 2010, the name was changed (once again) to Corpse Spiders. Although available in the gameplay demo, it wasn't one of the Witch Doctor's starting skills.
Skill tiers were removed shortly before the beta started in September 2011, and Corpse Spiders became a level 7 skill. The skill's mechanics were also changed when the name was changed, from a zombie that vomits spiders to a simple glass jar that is thrown at monsters and releases spiders upon impact. Flux was not impressed:
Whatever the delivery system, this one is fairly ineffective. The concept seems to be that this is a DoT spell that deals Physical damage instead of poison or fire or whatever is the ghost damage Haunt deals (Arcane). Unfortunately, there’s no reason (at least not in the beta) that you’d want to prioritize one type of damage over another, since all you care about is more damage against monsters that have negligible resistances. On that front Corpse Spiders fails, since the damage is very low and the spiders are slow to start delivering it.
The skill system was once again overhauled for the Beta Patch 13 in February 2012, and Corpse Spiders was classified as a Secondary skill and moved to level 3. It was reclassified as a Primary skill for Beta Patch 15; while its damage was nerfed, its mana cost was also lowered considerably, low enough to make it a spammable skill.
 Previous Versions
See the Witch Doctor skill archive for more details on previous versions of Corpse Spiders (or Spider Totem, or Spider Statue, or Carrion Spiders) and other Witch Doctor skills.
Corpse Spiders can be seen in action on video here:
You can find pictures in the Diablo III screenshot and picture gallery: