Cybernetics Core
Required to upgrade all Protoss aircraft and required to be able to build the stargate, the twilight council and the robotics bay.
Fleet Beacon
Allows the carrier and the mothership to be built.
Also upgrades carriers and void rays.
Forge
Required to upgrade protoss weapons, shields and armor. Also required to build photon cannons.
Gateway
Has the ability to be upgraded into a Warp Gate, which can "warp in" units across the map to any spot within the range of Pylons or deployed Phase Prisms. Each Warp Gate can only "warp in" one unit at a time and is subject to a cooldown period before it can use the ability again. In addition, newly warped in units will gradually materialize and be vulnerable for a period (like a newly warped in Protoss structure), and if the Pylon or Phase Prism is destroyed while the unit is being warped in, the unfinished unit will be lost.
Nexus
Builds Probes and Motherships. Also can boast production speed.
Pylon
Required to build structures and if destroyed any active structure around it will power down.
Robotics Facility
Builds Immortals, Phase Prisms, Observers and the Colossus.
Robotics Support Bay
This structure is needed to build Colossus.
Stargate
Required to build protoss aircraft.
Templar Archives
Required to warp in High templars not dark templars.
Dark Shrine
It allows Dark Templar to be built.
Photon Cannon
All purpose base defense that detects stealth.
Twilight Council
Contains upgrades for the Stalker and the Zealot.