Category: Bug, an acknowledged defect in the program
Category: Discussion (among developers trying to find consensus on a design etc.)
Category: Enhancement, a request to add or enhance a feature
Category: Long term goal. May be code-related or a meta goal.
Category: Refactoring. A purely internal refactoring, with no user-facing changes
Category: Regression, a bug that occurred on the dev branch
Category: TODO, specific item that needs to be accomplished in working towards a goal
Misc: On backburner - not rejected, but won't be tackled for a while
Misc: A good issue for newcomers to start with
Misc: Outside help is wanted (e.g. from another project)
Misc: Not enough info (yet) to determine if valid issue
Misc: Review from another developer is requested
Rejection: Duplicate, another open issue exists that matches this one
Rejection: Invalid, this is not a bug - it's expected behavior, the user's fault, etc.
Rejection: Rejected, the enhancement in question won't be implemented
Rejection: Bug will not be fixed (usually due to technical reasons)
Rejection: Bug report could not be reproduced