For me it depends on the size, severity and when it is found.
If it's found during testing of a planned feature, it goes back into development to be re-worked on, and discussed in the scrum.
If it's major, red alert, red-flag stuff then it probably needs it's own task to be inserted into the next sprint.
If during general testing a heap of smaller, ordinary priority bugs are found, I tend to make up cards that say "fix 5 outstanding bugs" (or whatever suitable number), assign it a small value and but it in the backlog with all the other tasks.
Don't get hung up on the terminology - just make sure they can be tracked, repaired and accounted for.