"Recently the Protoss have found that the ability to rapidly bring more fighting forces to war zones is invaluable. The Recall ability creates a tear in the fabric of space-time, and any friendly units that enter the swirling vortex will instantaneously appear next to the Arbiter that initiated the effect."
Recall is a spell cast by the Protoss Arbiter. It opens a 5x5 wormhole which teleports all friendly units in the wormhole to the caster. You cannot Recall allied units, only your own. You cannot Recall ground units onto spaces that is impassable to them normally. So you cannot Recall ground units into Water, Lava and so on. 
Recall is staple in PvT late game, as the instantenous transportation exploits the Terran's army's lack of mobility. In the PvP and PvZ match-up, Recall is less common. In both match-ups the high gas requirement of the Arbiter usually loses to the much needed High Templars. Also, these enemy's armies are mobile enough to catch up with your Recall, in which case your forces severed from your main army and are quickly taken out. In the rare case Recall is seen in these match-ups it is usually used for a tactical strike, such as taking out an island expansion.
Whereas Stasis Field usually is the prioritised spell when dealing with a Terran army, Recall may be researched if the Protoss is confident he can hold off the Terran push. A Recall can be devastating to a Terran main base, as key buildings can be taken out quickly. Recall is especially effective when dealing with island expansions as the Terran usually doesn't have the means to send a comparable force of reinforcements.
Considerations when recalling
Recall has a matrix of 138X133 Width X Height.
When recall is used in the center of this formation
It is seen that the outer line of zealots was not recalled. This is because they are only partially within the 138X138 boundary of recall. Units not entirely under this area are not affected.
In this one we can see that arbiter was able to recall a 138 Width of Zealots but only 128 Widht of Dragoons. This simply means that a unit has to be totally under the recall zone to be recalled.
Final note: Does not count the positions of unit before recall because after recall all positions will be mixed.
- Often the Terran makes sure a Science Vessel is patrolling his vulnerable areas. An EMP can deplete the Arbiter's energy, making a Recall impossible. Alternatively, Lockdown can be used to stop the incoming Arbiter in his tracks.
- When enough Turrets are in place, they can shoot down the Arbiter before it reaches the designated area. Even when the Arbiter gets off a Recall, it will be shot down, dispelling the cloaking field and making it easier to deal with the Protoss units.
- When expecting a Recall, Terrans do well to plant Spider Mines in their main. A Recall into a minefield will instantly turn the tide of the game as all mines go off instantaneously, massacring the Protoss forces.
In Combination with Other Spells
- Any status changes such as Irradiate, Lockdown, Parasite etc. remain active on Recalled units.
- Units that are Stasised or Burrowed cannot be Recalled.
- Hallucinations cannot be Recalled.
- Arbiters can't Recall themselves.
- Larvae and Eggs cannot be Recalled.
- Lurker Eggs can not be Recalled, but Cocoons can.
- Midas vs Draco on Gaia - WCG 2006 Group B - AND HERE'S THE RECALL.. oh no it isn't. You guys I'm sorry, I really thought he was gonna Recall - OH HERE IT IS!
- adapted from: StarCraft Compendium: Protoss Strategy <http://classic.battle.net/scc/protoss/special.shtml>