A system for discussion thread management using tag-based categorization, in which three types of tags are provided together to enable effective categorization of discussion threads, based on both automated and manual tagging. The three tag types are 1) user-defined tags, 2) moderator-promoted tags, and 3) system-generated tags. User-defined tags are arbitrarily defined by individual users and associated by users with posts to meet specific user needs. System-generated tags are automatically generated for each sub-thread to summarize a post and all the replies beneath it. System-generated tags can be promoted to a moderator-promoted tags when a moderator user verifies the correctness of a system-generated tag. The disclosed system detects topic changes in discussion threads based on the tags associated with each post in the thread. A moderator can manually examine sub-threads that have been determined to be off topic, and take appropriate actions. For example, a moderator may delete an off topic post or sub-thread, or fork a new thread of discussion for an off-topic sub-thread. Such processing of off-topic sub-threads can alternatively be made by the disclosed system automatically, without moderator intervention. Moderators can also define policies to control discussion moderation based on the disclosed tags and off-topic determinations.