This is for Minor problems, such as failures at extreme boundary conditions that are unlikely to occur in normal use, or minor errors in layout/formatting. These problems do not impact use of the product in any substantive way.
This is a problem that a) Effects a more isolated piece of functionality. b) Occurs only at certain boundary conditions. c) Has a workaround (where "don't do that" might be an acceptable answer to the user). d) Occurs only at one or two customers. or e) Is very intermittent
This should be used for only serious problems, effecting many sites, with no workaround. Frequent or reproducible crashes/core dumps/GPFs would fall in this category, as would major functionality not working.
This should be reserved for only the most catastrophic of problems. Data corruption, complete inability to use the product at almost any site, etc. For released products, an urgent bug would imply that shipping of the product should stop immediately, until the problem is resolved.