Rushing

Rushing is the process of using a higher level character to "rush" or "turbo" one or more lower level characters through the game. This is a useful tactic in most RPGs since characters earn higher rewards from higher level monsters.

For example, it might take an hour or two to rush a low level character up to a higher difficulty level, and after an hour or two of play time there (sharing in the experience of the kills made by other players) the low level character would have advanced to a high level that would have required many hours or days of play to reach normally.

It's not yet known if rushing will be permitted or beneficial in Diablo III.

Rushing Etiquette

It's common for low level characters to offer to give up the item drops or various quest rewards, in exchange for being rushed. It's very poor form to renege on this sort of agreement, but in the cut throat world of online gaming, it's not unheard of.

Rushing in Diablo II

Rushing was very common in Diablo II, since the shared experience at higher levels was vastly greater than what any low level character could earn playing for real. Also, many eventually grew bored of running through all of the quests and killing all the low level enemies, and wanted to jump right to the end game, where the best items could be found.

There were also a variety of nice quest rewards that characters obtained along the way. Rushing low level characters through Act Four of Nightmare and Hell in order to obtain the Hellforge rune drops became a very popular past time once the quality of those runes was increased in the v1.10 patch.

Rushing Limitations

Most games have various limitations imposed to stop or limit rushing. Characters outside of a certain level range can't share in experience, or pass certain quests, or take certain waypoints, etc. Developers generally try to limit rushing to keep it from being so easy that no one plays the early levels of a game, but they realize that players will eventually want to hurry through some parts of the game, so they don't stick minimum level requirements on every area or act.