Overview
Rushing is generally considered a low-level strategy, primarily because advanced players are often prepared to resist a rush and because rushing causes the player to fall behind on their economy at the beginning of the game, which would ultimately result in an enemy victory if the rush is not successful.
Rushing does not have to be totally successful--taking out an enemy base completely--to be rewarding. If a handful of rushing units can dispatch multiple enemy workers, then the enemy economy will suddenly be in an even worse condition than that of the rushing player. The most efficient way to dispatch as many workers as possible is to order a regular move to a spot near the worker line, then hold shift (used to queue orders) and order an attack response in a random spot in the middle of the worker line. This way, even if your opponent has built a decent defense, your units will run past the base defenses and immediately start attacking the workers, with little requirement for micromanagement on your part. A common mistake of amateur rushers is to order the units to the opponent's base in attack response mode. This works if your opponent has little or no defense, or if you've already destroyed the base defenses and military units. Otherwise, the rushing units will do minimal damage, and the opponent will mostly recover and you will fall behind in economy and be at a serious disadvantage. The best option for seriously crippling an opponent is to aim for the workers and the unit production buildings.
In order to succeed, however, rushes must be conducted swiftly, either by quickly obtaining a handful of low-level units at the expense of economy and technology, or by quickly obtaining a handful of high-level units at the expense of both economy and all other military. Should the enemy defenses complete before the rush, the rush would either fail or be called off, and in each situation the rushing player would be at a loss.
Good recon is the cornerstone of any successful rush (see the Scouting section of the Wikibook for more information). A good recon strategy, once you get around your eighth or ninth worker, is to send one of those workers to the enemy base (or, if you're Zerg, send your Overlord). At his point, the other player will probably be either 1) building more workers and supply (Supply Depot for Terran, Overlord for Zerg, and Pylon for Protoss) to bolster his/her economy, or 2) starting his/her basic military production facility (Barracks for Terran, Gateway for Protoss, Spawning Pool for Zerg to allow production at hatchery). The ideal would be to position your worker so that you can observe their production strategy undetected; with the worker's short line of sight this may not be possible, but if your opponent isn't watching his base carefully, he may miss the worker somehow. Of course, be sure to continue the production of workers and units during the worker's trip there, and don't get too preoccupied with micromanaging the recon mission. If, somehow, you observe that your opponent has built a superior defense and/or military force, do not be foolish and force the rush; instead, tech up a bit and/or build more units, and go for an early middle/mid game rush. But, if you believe that you can catch your opponent off-guard, go ahead with the early game rush. As with any rush, timing and speed is essential.
In order to properly resist an enemy rush, it is best to have either one defensive structure protecting the base and mining operations or a handful of units to repulse an enemy attack. The former is cheaper, because defenses are inherently cheaper than normal units, but is also inflexible, especially when the enemy scouts can report what structures a player is building and the enemy can thus choose to expand instead of obtaining military units. In other words, the enemy would be forced to obtain units for each of your units, while the enemy would likely create one expansion for about every three defensive structures, because the enemy would never fear an attack by your static defense (they DON'T MOVE). If your opponent's rush fails and you still have a handful of units, a possible strategy is to execute a counter-rush. Being that the enemy rush failed, your opponent will likely have lost most of his/her units, which you can confirm by recon (see above). If this is indeed the case, take advantage of it and try and strike back. However, don't be too quick to try this if you haven't conducted proper recon and you're not sure what's happening at your opponent's base.
For the remainder of this article, please see Rush Types.