We classify our bugs and defects according to both their priority and severity.
The priority level is an indication as to how urgent it is to fix/correct the problem (urgent, high, medium, low, none).
The severity level, helps us identify how much or what kind of damage can be caused by the defect (dangerous/destructive, degraded and no workaround, affected but workaround exists, nuisance/cosmetic, no impact).
Typically, the more dangerous and destructive the bug is, the higher the priority. However, it is not guaranteed. Consequently we can wind up with the occasional bug listed as dangerous and destructive, but due to the rarity of the situation, or the amount of change that may be required to fix it, its priority can in theory become quite low.
Hope this helps.