"OMG Zerg Rush!!" -The typical reaction to a Zergling Rush in a multiplayer Starcraft game
Zerglings are the basic military units of the Zerg Swarm. They rely on their amazing speed in combat and attack in large packs. They are the easiest Zerg unit to amass because Zergling eggs always produce two Zerglings at a time.
The Zergling was assimilated from the Zz'gashi Dune Runner, and one of the first strains created for the swarm after the Zerg departed from Zerus. Zergling embryos are housed within Spawning Pools, which are then taken to larvae when ordered to spawn Zerglings. They have great speed, which makes them great for scouting and hit-and-run attacks. They attack by charging at enemies and rending them with sharp, hooked claws.
Tactics
Using Zerglings: Zerglings have a low creation time and Zergling eggs always spawn two at a time, so it is easy to create a huge Zergling swarm instantly with at least three Hatcheries/Lairs/Hives. Zerglings are fast, so they make good scouts, and they can use this to attack and then retreat to safety. Zerglings can ambush enemies by burrowing underground and then bursting from the earth when enemies draw close enough. Another tactic, called the "Zergling Rush", involves spawning a massive Zergling swarm early in a match, then sending them to an enemy base. At best, the enemy has virtually no defenses when the rush takes place, and is easily wiped out by the swarm. At worst, there will be a good amount of enemy troops and/or defense structures, shifting the balence in favor of the enemy.
'Fighting Zerglings:' Zerglings are fast and can shred their enemies quickly, but their speed comes with the cost of weak defense. Their armor is very light, and they can easily be killed with a large enough group of basic infantry, like Marines and Zealots(or even other Zerglings!).
Stats
Armor: Light
Weapon: Claws. Melee type. High attack speed.
Upgrades: Burrow(Hatchery), Melee Attacks(Evolution Chamber), Armor Carapace(Evolution Chamber), Anabolic Steroids(Greater movement speed) (Spawning Pool), Adrenal Glands(Greater attack speed) (Spawning Pool)