Zerg ground units on Creep receive a 30% movement speed bonus, with the exception of Drones with no bonus, Hydralisks with a 50% bonus, and Queens with a 170% increase. Zerg buildings will take 2 damage per second until destroyed unless Creep is present. Creep is not owned by the player that produced it, so these bonuses and damages will take effect regardless of the player that made the Creep (e.g. Zerg units get a speed bonus on enemy Creep).
As zerg structures are effectively giant organs and all of them rely on Creep for nourishment (apart from the Hatchery and the Extractor, the former growing creep), creep effectively makes any zerg colony a single living creature, sending its tendrils deep into the earth in order to absorb raw minerals and process it into a nourishing substance. In addition to being spread by these structures, creep is able to be spread by Overlords. Once established, the spread of creep can easily be observed in real time.
Creep itself is akin to a living organism, and an extremely durable one at that. It possesses a self-replicating cell structure, is extremely resilient and is capable of proliferating in any environment; even in a deoxygenated environment or over water. Despite this resilience, it is a rather brittle substance and if creep is over hollow ground, an individual is doomed to fall through. Its only vulnerability is extreme heat; it will not grow over hot material and high explosive rounds are able to set creep alight. However, since creep is capable of near instantaneous generation, incineration is not as effective as one may hope for. The only guaranteed method to dispose of creep is to destroy the hatchery and/or creep colonies fueling it. Confirming that all colonies are destroyed is not easy however and creep can look "dead" one second and begin moving the next.
Creep is of a purple coloration and feels thick and slimy. Like the zerg species as a whole, creep is sensitive to psionic powers; so sensitive that some psionically gifted individuals can even control it.