Overlord

Overview

A mutation of a semi-intelligent spacefaring animal species known as "Gargantis Proximae", the floating Zerg strain known as the Overlords are essentially psychic dispatching posts, relaying the orders of the commanding Cerebrates to the lesser Zerg strains, such as Drones, Zerglings, and Hydralisks, just as the Cerebrates relay those orders from the Overmind (or later, from Sarah Kerrigan) to the Overlords. 
 
A single Overlord can only keep track of so many units, however, and so the Zerg rely on a Control system just as much as the Terrans rely on Supplies and the Protoss rely on Psi. As a Zerg army grows, its Cerebrate must continually spawn more Overlords to keep all those creatures in tight rein. As such, any Zerg base is likely to have at least a half-dozen Overlords hanging around it, with more likely wandering the map.  
 

StarCraft

 

The Overlords found in a new hive are very, very slow,  and have no attack and no abilities, beyond being Detectors, which allows them to see cloaked or burrowed enemy units. While the hive grows, they are often used to scout out the area immediately surrounding it. Various evolutions are available to make them more useful as the hive grows; Antennae increase the range of their sight, furthering the distance they can detect invisible units, a Pneumatized Carapace increases the movement speed of an Overlord, and while they're still very slow, they are at least able to keep up with Terran Dropships - which is handy, because the third major upgrade, Ventral Sacs, allow them to carry ground-based Zerg strains within their body and transport them across the map, essentially making them living Dropships. Finally, they also benefited from the Flyer Armour upgrades at the Spire, which allows the Zerg to build air units.  
 
Specs: 

Role: Airborne commander/detector and transport unit 
Resources: 100 minerals 
Build time: 40 sec 
Control: 8 units  
Sight range: 9 (11 upgraded)
Detection range: 9 (11 upgraded) 
Hit points: 200 
Armor: 0 (1 per Flyer Carapace upgrade)
Transport: 4 units (with Ventricle Sacs upgrade)
Evolves from: Larva 
Hot key: O

 

StarCraft II

Like many other units, the Overlord saw fairly significant changes to its abilities and upgrades from StarCraft to StarCraft II. Though the basic mineral, supply cost, and all aforementioned upgrades remain mainly unchanged (with the exception of the Antennae upgrade, which they no longer require), they lose their Detector status and gain the Excrete Creep ability, which causes the Overlord in question to cover the ground directly beneath it with Creep, the substance upon which most Zerg buildings must be built. This has no energy or time limit, and works as long as the Overlord isn't moving. This allows them to allowing for further tactical options through mobile creep generation, such as surreptitiously building an array of Spore and Spine Crawlers outside an enemy base. 
 

The other, larger, change came in the form of the Overseer Morph ability. This causes the Overlord to retreat into a hovering cocoon and transform into a new unit, the Overseer. Overseers cannot transport ground units or excrete creep, but re-gain the old Detector status; an inherent ability that is only shared by the Spore Colony and Spore Crawler. Blizzard commented that this change was because Zerg players tended to have many, many Overlords around, giving them the ability to cover wide areas of the map with Detectors, rendering the Zerg too "Detectoriffic", in their words. Overseers can also spawn Changelings, another new unit. Changelings have a limited lifespan, and when coming near enemy units, will shapeshift into an accurate recreation of a friendly Marine, Zealot, or Zergling, depending on whether that enemy is Terran, Protoss, or Zerg. 
 
 
Specs: 

Role: Airborne commander and transport unit 
Resources: 100 minerals
Build time: 25 sec 
Control: 8 units  
Sight range: 11
Detection range: 11
Hit points: 200 
Hit point regeneration per sec: 0.2734
Armor: 0 (1 per Flyer Carapace upgrade) 
Speed: 0.4687 (1.8749 with Pneumatized Carapace)
Evolves from: Larva 
Evolves into: Overseer
Hot key: V