A rush is a powerful attack very early in the game. By performing a rush, the player typically sacrifices late-game stability in favor of a strong early army. While not cheese, rushes still leave the attacker in a weak position. If the defender can stave off damage, your opponent will be in a much more secure mid-game.
If scouting reveals a fast teching opponent, the attacker may decide to move for an All-In attack, committing most or all of their workers to the attack to supplement whatever offensive units were produced.
A rush can be considered successful if any of the following are accomplished:
- Win outright
- Force the defender to overcommit to defensive units or structures
- Kill all or most workers
It should be noted that even if the attacking force is completely wiped out, a rush can still leave the attacker in a better position if enough damage was done to the defender.
Here are a few articles as examples of a rush