The enigmatic Protoss are by far the most powerful race on a unit-to-unit basis. Their sturdy plasma shields and strong attacks are the key differentiating factor in the race, but these advantages don't come without a hefty price tag. In general, Protoss units are far more expensive and slower to build than units of the other races. As such, proper planning of upgrades and build order are essential for maximum efficiency.
The 'rush' is a technique that cen be employed by any of the races against any of the other races -- if it is properly done. In general, a rush is any attack on a base that happens within the first five minutes of the game. Its intent is to destroy any enemy outright before he gets really entrenched, or at the minimum, destroy enough infrastructure (primarily workers) so that his development is greatly slowed.
As the Protoss, you generally develop more slowly than the other races. That said, the Protoss can be the most devastating rushing race in the game as it only takes a few units to break the early defenses of other races if your opponents aren't prepared (and more often than not, they aren't). Given their high speed and toughness, the primare instrument of your rush will be the Zealots. They do a considerable amount of damage to structures and a pair can mow through loads of workers and most modest defenses quite handily.
The decision to rush should be based upon a number of factors. First and most important is, what sort of game is it? Is it teams? Is it one-on-one? Is it free-for-all? In a free-for-all game, rushing with the Protoss is generally not a good idea given that the likelihood of one of the other players rushing you is pretty good. Leaving yourself open for one of several other players probably isn't a good idea just going by the numbers.
In a one-on-one game, the decision should be made based upon three factors: