The untagged tag is used by the system to mark any question that does not specify any other user tag. A question can become untagged (loose all his applicable tags) for many reasons, including migration from an other stackexchange site or because original tag was deleted.
If you see a question marked as untagged please help us to get rid of it by by retagging it.
Please refer to this meta topic for further details about the "Help us clean up the untagged tag" program.
