Alpha, in the context of Guild Wars, refers to the early phase of development and testing of future campaigns and game updates for already released campaigns.
This is done by ArenaNet together with a group of selected volunteers who assist in testing, bug-searching and balancing discussion.
Unlike beta testing, which is "(semi-)open," alpha is strictly "closed," meaning alpha testers are required to sign a non-disclosure agreement (NDA) with ArenaNet.
An early alpha test version of Guild Wars