Pasted

'A' bug -- The 'A' bug is the very worst kind of bug. This type of bug can be summed up thusly: "It would be an unthinkably bad disaster if the game was released with this problem unfixed." Some examples:
   - the game crashes;
   - there is a virus in the game;
   - there are obvious spelling errors;
   - there are obvious graphical or audio problems;
   - a feature (in a menu or accessed by pressing a button) does not function;
   - there is no copyright language in the game anywhere;
   - the game is not fun to play.
Releasing a game with this sort of flaw would generate very bad public reaction and bad press, or there could be legal ramifications against the company.
 'B' bug -- The 'B' bug is a major bug. It's not quite as bad as the 'A' bug. It can be summed up thusly: "It would be unfortunate if the game was released with this problem unfixed, but the game is good regardless." In a pinch, if the company has a need to release the game and stop spending money testing and fixing it, and if Customer Support, Sales, Marketing, QA, and the executive staff all agree, the game may be released with minor flaws. For example:
   - bugs which do not ruin the experience of playing the game;
   - noticeable graphical or audio problems (especially if you know where to look for them);
   - highly desirable features were left out (and are not mentioned anywhere in the game).
'B' bugs will likely show up in press reviews of the game but are things that are probably hard (expensive; time-consuming) to fix. The playing public won't be happy with these problems, but the overall playing experience is not ruined by the existence of these problems.
 'C' bug -- The 'C' bug is a normal bug. The tester may feel strongly about this problem s/he has identified, but when weighed against the company's larger need to release the game, the bug isn't that big a problem in the decision-makers' view. When push comes to shove, 'C' bugs may have to fall by the wayside (if they're hard to fix, that is -- a 'C' bug that's easy and quick to fix is likely to simply get fixed, unless the project is coming down to the wire).
 'D' bug -- The 'D' bug is a minor bug. Often one hears something like "It would be nice to add this feature." Especially when reported later in the test process as the release date nears, 'D' bugs are likely to remain unfixed.