(Taken from wikipedia):
"Beta" is a nickname for software which has passed the alpha testing stage of development and has been released to users for software testing before its official release. It is the prototype of the software that is released to the public. Beta testing allows the software to undergo usability testing with users who provide feedback, so that any malfunctions these users find in the software can be reported to the developers and fixed. Beta software can be unstable and could cause crashes or data loss.
Closed Beta is a phase in which only a few selected individuals are given the chance to test the software, contrary to an Open Beta (which usually comes after a Closed Beta), in which a larger number of users are allowed to participate.
More specifically, for Dragon Ball Online:
Closed Beta Edit
During the second half of 2009, Netmarble Korea conducted 3 Closed Beta's for Dragon Ball Online, officially they referred to each Closed Beta as a Premium Test.
The first Closed Beta became active in September on the 18th (6pm to 11pm), 19th (4pm to 11pm) and 20th (4pm to 11pm).
Second Closed Beta became active in November on the 5th (5pm to 10pm), 6th (5pm to 10pm) and 7th (2pm to 10pm).
Third Closed Beta became active in December on the 9th at 4pm and finished on the 13th at 10pm.
Open Beta Edit
On December 29th 2009 , Netmarble Korea announced that a Guerrilla test will be on January 9th 2010 starting at 2pm and finishing at 6pm. On the same day, they also announced Open Beta will start on January 14th 2010.
* Note: The times displayed are set in Seoul time. (GMT +09:00)