Beta

In the context of this subject, Beta refers to the timeperiod before the game went "live" (i.e. before the game was publicly sold at stores). It is the time period where people played on the servers that ArenaNet had provided in order to test the program itself for the company while at the same time, give players a taste of the game (in not always convenient bite sized weekend chunks once a month) in order to stir up sales. Sometimes called (mostly by the company) as a World Preview Event or WPE.

For more general information see: "Beta Version" on Wikipedia