Creep is needed to place Zerg buildings and also causes all Zerg ground units, excluding burrowed units, Drones, Broodlings, and Changelings not disguised as Zerglings to move faster when traveling over it. The increase is 30% for all units, barring the Queen (166.67%), Hydralisk (50%), Spine Crawler, and Spore Crawler (150%). If a Zerg building is not on creep, it loses health at a linear rate of 2.73 health per second, a combination of the base health loss rate, 3 hps, and Zerg Regeneration of 0.27 hps.
The speed bonus makes it beneficial for Zerg to cover the map over which they are moving in creep, so as to increase their mobility and their ability to reinforce besieged bases or retreat from indefensible positions. Because the creep bonus applies to enemy Zerg as well, it is common in ZvZ to see creep production restricted only to areas around the Zerg base to avoid facilitating enemy Zerg rushes.
Creep itself does not grant vision. Everything that generates creep gives vision, but sometimes that vision does not extend as far as the creep itself does. For example, the Hatchery's vision range is one less than its creep range.
Lair and Hive do not increase the creep range, but increase the vision range +1 each time. With Lair the enemy can still barely step over the edge of the Lair's creep and not be seen. With Hive the vision goes beyond its creep range, so a unit could be spotted even off creep, if they are near it.
Protoss and Terran buildings (except Auto-Turrets) cannot be built or landed on Creep. Zerg may cover the future site of a Protoss or Terran expansion with Creep, forcing their opponent to destroy the source of the Creep then wait for it to recede before the opponent can take the expansion.
Creep spread is very important against Terran due to the prevalence of melee units (Zerglings, Banelings, and Ultralisks) in the matchup. Creep allows the army to engage much more effectively against the ranged Terran units. Creep spread also grants vision of potential drops and forces the Terran to use scans or risk being forced into an unfavorable engagement.
Creep spread from Overlords is often used to prevent Command Centers from landing at third and fourth base locations. Proxy hatchery builds at the natural expansion are occasionally used to prevent the Terran from taking their second base. Such strategies, when carried out ideally, can force multiple scans through spread out Creep Tumors and consequently delay the expansion for an extended period of time.
The mirror matchup is the scenario where creep spread is the least important since the movement speed boost is granted to both sides. However, although spread into the map is not essential, it is still important to spread creep to the third base to allow for defensive movement, especially for Queens.
- Building a Hatchery. Hatcheries generate creep on their own, and are required for building placement early game. Note that it is possible to cancel a Hatchery and use the ensuing creep to build proxy buildings or secret tech. As the creep will quickly expire, buildings left without creep will start to die. You can however place a Creep Tumor on the creep to maintain and expand it.
- Overlords. Overlords in StarCraft II have the ability to "Generate Creep" after upgrading Hatchery to Lair, this ability costs nothing as it is not an upgrade and simply an off or on state of generating creep. The generated creep encompasses a 2x2 area directly under the Overlord. When the Overlord moves or Generate Creep is switched off, the creep will slowly disintegrate.
- Queen's "Creep Tumor" ability. The Queen's "Creep Tumor" ability costs 25 energy and spawns a "Creep Tumor". This "Creep Tumor" excretes creep and has a single charge to make another Creep Tumor. Creep Tumors are treated as structures and have 50 hp. They are also invisible, so one needs detection to be able to kill them. Once a Creep Tumor has been destroyed, the Creep will slowly recede.
- Creating a Nydus Worm will also generate a 4x4 area of Creep around it after emerging from the ground.
The "4x4" area, and "2x2" area notations used above are technically both misnomers. Creep actually generates in a "Squared Circle" pattern, with a set radius in distance units. A distance unit is the smallest whole unit defined internally in Starcraft. The length of a distance unit is approximately the length of a Zealot. A square unit is a 1 Distance Unit by 1 Distant Unit square area. This is approximately the total space a Zealot takes up.
As the buildings which generate creep are all an odd number of units in each dimension and must be placed on the building grid while the creep generated has an even dimension, the creep generated by Hatcheries, Nydus Worms and Creep Tumors is not centered upon the building, but is instead shifted one square unit down and to the left. This leads to balance issues on certain maps where more Creep Tumors are required to extend a path from the starting creep to the bottom of the ramp in some starting positions than others.
A Creep Tumor generates creep in a 10 square radius squared circle. This means it generates creep in a total of 292 squares. An Overlord generates creep in a 6 square radius squared circle. This means it generates creep in a total of 96 squares.
- Creep doesn't start being generated until a delay period. On Overlords, this is 2 seconds. On Creep Tumors, this is 0 seconds.
- Creep is generated every Creep Generation Period. The Creep Generation Period of Overlords is 1 second. The Creep Generation Period of Creep Tumors is 0.8332 seconds. At the end of each of these periods, just less than 3 square units of creep are created on average (about 2.835, based on observations detailed below).
- Creep is generated randomly, so long as it is adjacent to existing creep. This is how the cool growth effect works internally. This has a hidden ability in it though: Multiple creep sources generate simultaneously.
- An Overlord takes about 34 seconds to fill up its entire area. Judging by how its creep generation is defined, 34 seconds is likely the exact value.
- A Creep Tumor takes just less than 86 seconds to fill up its entire area. Judging by how creep generation is defined, it is likely 85.8196 seconds exactly.
Optimizing Creep Growth
The easiest and quickest way to optimize creep growth is to add more creep sources. The time it takes for two Creep Tumors to fill an entire area is about 43 seconds. The time it takes for three is about 29 seconds. It's obvious from these numbers that there are diminishing returns.
When you generate creep edge to edge, you tend to save 124 square units. This translates into saving about 36 seconds on one tumor. On two tumors, the amount of time saved is actually halved, since you would have generated it twice as fast. Thus you save only about 18 seconds, and it takes about 25 to fill. With three, you would only have saved 12 seconds, and it would take 17 to fill, etc.
As the Spawn Creep Tumor cooldown is now 30 seconds, it is never worthwhile to have three Creep Tumors together. Two, however, can noticeably increase the speed of creep generation.
The other main way to optimize creep growth is having multiple paths. You do not need a single line, and generally it is beneficial to have creep covering as much of the map as possible. This obviously increases the amount of micro required to keep the creep up, but each path basically consists of the problem above.
As explained by oesis here creep can be spread quicker if it is generated near an impassible border. The creep looks for a location without creep and fills it, by placing it near a wall you eliminate the areas that it can spread thus speeding growth in a specific direction.
For additional creep mechanics check out this discussion.