Sync enter
From Guild Wars Wiki
A sync enter, sync join, join spike, syncing, spike join, or team stacking occurs when two or more players attempt to join the same team of a "randomly" selected PvP arena (i.e. RA, AB, Dragon Arena, Rollerbeetle Racing) team. It is controversial because it sidesteps the nominal randomness of teams and can create highly skilled teams that are often difficult for non-sync teams to defeat.
Sync enters, more often than not, take place in the International or Asian Districts where the player count is very low. Since fewer people are joining the type in that specific type of district (not the district number), you are more likely to enter at the same time as a friend/acquaintance and be on the same team.
Some players attempt to disrupt sync entering, by entering those specific districts, because teams are not able to sync efficiently when other players join the queue. Very organized sync enters are not greatly affected by this however as they simply coordinate over voice chat programs such as TeamSpeak.
Non-controversial syncs also occur. In PvE syncs are useful for certain missions, i.e. Vizunah Square and Unwaking Waters when teams on both sides wish to play with human counterparts. Players working on the Canthan cartography title often sync entered The Jade Quarry because there were typically not enough players to start a match, but since a rewards update, this is no longer the case.
[edit] Notes
Gaile Gray has stated multiple times [1] [2] that syncing is not considered a cheat or exploit by Arenanet.
Linsey Murdock, in response to the comments above, has stated that Gaile Gray is no longer in the position to give the best answer to this type of question.
Linsey Murdock states on her FAQ page [3] that ArenaNet is investigating how to fix Sync issues.
In the September 1, 2009 update, syncing was supposedly "fixed". However, it's still possible to sync in random PvP, but it is more difficult than it was before the update.

