The Probe is the Protoss worker. It harvests Resources, has a low-powered ranged attack (with a range of one), and can build any Protoss building. All buildings other than a Pylon, Nexus, and Assimilator must be built within power range from a nearby Pylon. Unlike the Terran SCV, the Probe does not have to keep working on a building being warped in, and instead can go do other tasks such as build more buildings or harvest resources. Two factors that set apart the Probe from its Terran and Zerg counterparts are its regenerating Psi Shields and its quick acceleration. Shields play a very large role in the scouting and harassment abilities of the Probe, as a skilled player can avoid pursuing units(such as Zerglings or Marines) well enough that any damage to the shields will be quickly restored. The Probe's faster acceleration when compared to Drones or SCVs gives it an additional advantage in regards to longer lifespan in the opponent's base. Other common uses stemming from these attributes are delaying Hatchery/Pool placement and harassing building SCVs.
See also Basic Unit Commands
Used to scout the Zerg base, probes are often used to deny used to deny the opposing Zerg's expansion attempt by harassing the expansion drone and/or placing a Pylon at the Zerg's natural to block Hatchery placement. Bisu is famous for his ability to keep his scouting Probe alive in the Zerg base for a long time, often up until his Corsair is able to take over scouting. A scouting Probe can be used to delay the Zerg's expansion to its natural by moving in the way, and/or placing a Pylon at the Zerg's natural to block Hatchery placement.
In the event of early aggression by the Zerg, Probes can be called off of the line to defend. This includes surrounding Photon Cannons to prevent Zerglings from attacking them and blocking choke points to prevent Zergling run-bys. If the Protoss didn't fast expand and instead opted for early aggression, probes also can be used in conjunction with Zealots, as probes have a ranged attack that enables them to attack from behind the zealots and deal extra damage.
The probe may also be used in conjunction with a Zealot to snipe a scouting Drone low on health that passes near the mineral line. Sometimes if a Protoss is doing a Zealot rush he brings a few Probes along with the Zealots. This is because one Probe allows one Zealot to kill a Zerglings in one less hit.
Used to scout the Terran base, the Probe can be also kill SCVs in the process of construction in order to delay the Terran. Probes can also place an Assimilator at the Terran's main gas (Gas Steal) to delay the Terran's Factory and further tech. Occasionally, the Probe will be used to place a Manner Pylon, but this is less effective against Terran than against Protoss due to the SCV's bigger size and stronger ability to glitch out.
Used to scout the enemy base, Probes are sometimes used to place Manner Pylons. They can also be used to place three Pylons in a V formation in front of an enemy's Gateway to trap any Dragoons coming out of the Gateway (this is referred to as a Pylon Prison).
Both +1 armor and +1 shields give Probes an extra hit against Dark Templars.