Mutaling is a Zerg strategy for Starcraft. Also called Korean Zerg because the play style is the most popular there, the primary focus of the strategy combines mutalisks and zerglings to their maximum efficiency.
The build order goes something along these lines...
12-hatchery at chokepoint or expansion
16-upgrade to lair
The primary goal is to obtain mutalisks as quickly as possible- while the lair morphs, you need to save 200 gas to construct the spire. Some players obtain evolution chambers or get the speed upgrade for the zerglings, for fun and profit, but the main goal is to get six or more mutalisks as you spawn a number of zerglings.
When going after your opponent with your forces, the mutalisks should be sent in first, and the zerglings should follow quickly after them. This setup usually forces static defenses such as photon cannons and units that attack both air and ground targets to focus fire on the mutalisks- and while the mutalisks absorb the damage, the zerglings come in from behind and begin wreaking havoc. After the intial attack, the mutaling player can either start assuming map control by expanding a number of times, or they can renew the assault and finish their opponent off. Muta harassment also factors into this strategy as a secondary option.
While this technique is often painful for Protoss players who haven't reached Templar tech and can do significant damage to another Zerg player who hasn't been keeping up with your upgrades, it often falls hard against Terran players. M&M can shred mutalisks before the zerglings can get into place, and rip through zerglings as a general fact. Mutaling also falls flat against opponents armed with Psionic Storm and is hurt badly by lurkers.