The overlord may evolve into the overseer which is a detector and spellcaster. Overlords are commonly used in the early game to scout areas and gain information on what strategies the opponent is going and what route they attack from. Overlord positioning is key to playing zerg, however picking off overlords can supply block the zerg player. They can commonly be hunted down by cheaper air units such as mutalisks, phoenixes and vikings, as well as roving packs of terran infantry.
When Kerrigan reaches a certain level overlords provide more control and are spawned instantly. This ability is called Improved Overlords. Overlords in the campaign lack the ability to transport units.
All zerg commanders (except Dehaka) use overlords for control. All commanders can upgrade overlords with pneumatized carapace and morph to a overseer Kerrigan, Zagara and Abathur Overlords use the ability a create creep and transport function. Zagara's overlords use the Leviathan Brood's skin.
The following section contains information from a previous version of StarCraft II which is no longer valid.
The reason for the changes and the split between the overlord and overseer in terms of detection was that the zerg "were somewhat 'detectorific'" in StarCraft.
Overlords regained the ability to transport units as of December 2008.
Overlords used to have a slime ability, but that has been removed from the latest build.
Slime (also known as "Corrupt Minerals")
The overlord could drop a non-attacking slime creature, temporarily disabling resource nodes and xel'naga towers. The minerals can be covered as they're being harvested, enabling raids which slow down an enemy's economy. The creature must be killed before the minerals can be accessed.
This ability took some time to "cast" and had a small cooldown timer.
Before Legacy of the Void, all overlords were upgraded to transports with the Ventral Sacs upgrade. The expansion changed the upgrade to effect individual overlords.
↑ 6.06.16.26.31) The tier 1 Overlord is very slow and does not have detection, though still essential for scouting throughout the game.
2) They are two different abilities. In generating creep, the Overlord essentially unloads a bunch of creep straight to the ground below it, which expands. Corrupting minerals takes some time to fully cast, but it creates a non-attacking creature that takes over that resource, and needs to be killed before those minerals are able to be gathered. Furthermore, you can corrupt resources that are already being mined, introducing potential Overseer raids to slow the enemy's production.
3)Neither abilities require energy. Creep generation does not have a cooldown whereas corrupt minerals has a small cooldown timer. Karune. 2008-04-25. Overseers having Creep Drop, details?. Battle.net StarCraft II General Discussion Forum. Accessed 2008-04-25.