The Photon Cannon, often simply called a Cannon, is a versatile Protoss defensive building capable of attacking both air and ground targets with a ranged attack (range=7). They are also detectors for the Protoss, thus often used to counter cloaked opponent units. Cannons first require a Forge to be built.
Cannons do not build any units, nor have any research upgrades. Their Plasma Shields, however, can be upgraded at the Forge.
Cannons are similar to the Terran Missile Turret in its detection and anti-air attack abilities, but more versatile in its ability to also target ground units. Cannons can also be compared to a combined Zerg Sunken Colony and Spore Colony in its ability to attack both air and ground units.
Because of their relatively inexpensive cost, it is common for the Protoss player to build multiple Cannons at vulnerable positions. These include at the top of a ramp, at chokepoints, and surrounding the Nexus at expansion bases. The range attack feature is of great use in dealing damage to non-ranged units (e.g. Firebats) -- cannons are sometimes placed in spread out positions to defend a larger area while requiring the non-ranged opponent to move from Cannon to Cannon, incurring damage along the way.
In team play, it is a common to place patches of Cannons to secure the middle of the map or at the chokepoints of the opponents as a dividing tactic. This is effective in the early to mid game in keeping opponents trapped inside their bases, rendering them powerless in defending their teammate(s). In team play, it is also common to see the Protoss player constructing Cannons at the chokepoint or base of the player's teammate. This tactic is helpful in defending against a teammate who is, or anticipated to be, the focus of the opponents attack. This provides cloak detection, defensive ability, and obviates the need for the Protoss player to station valuable troops at the distant teammate's base.
Cannons are very useful in most Protoss match-ups in defensive schemes. Additionally, Cannons are sometimes use as part of a Cannon rush. For example, it is common to see the Protoss player warping Cannons in the mineral line of a Zerg opponent's natural base in the very early game.
In the PvZ match-up, the Cannon is vital in build orders such as the popular Fast Expand strategy. In such builds, the Protoss player relies on Cannons to fend off Zerglings, Hydralisks, and Lurkers while climbing the tech tree and building an army. It is vital to build a robust Sim-City defense in these build orders, as the Protoss player can be vulnerable to Zergling run-bys which then target any constructed Cannons. Cannons are also common in defending the mineral line against Mutalisk attacks during the mid-game; such Cannons can be supplemented by Archons and High Templars with Psi Storm. If the PvZ game reaches the late game with multiple expansions, large patches of Cannons are commonly built to surround the expansion Nexus, but this tactic is countered by Dark Swarm use.
In the PvT match-up, the construction of Cannons is often delayed until the mid game as the Protoss player typically favors mobile detection via the Observer and opts to spend resources on Dragoons, instead. As such, the Forge and Cannons are usually not built until the third or more expansions are started, at which point several Cannons are placed at chokepoints or surrounding the expansion Nexus to defend against Vulture harassment. Large numbers of Cannons are rarely built against Terran due to the strength and range of the Seige Tank.
In the PvP match-up, the construction of Cannons is also often delayed until the mid game as the Protoss player typically favors mobile detection via the Observer and opts to spend resources on Dragoons, instead. Cannons are often built to defend against drop-play by the Protoss opponent, particularly for Reavers, High Templars, and Dark Templars. If the PvP game reaches the late game with multiple expansions, large patches of Cannons are commonly built to surround the expansion Nexus.
In Korea, Photon Cannons are sometimes referred to as "Flowers" due to their shape. Large groups are therefore dubbed "Flower Fields" and a particular player may be dubbed "Flowertoss" for prolific use of "Flowers."