There are several important pieces of information in a bug report. The version containing the bug, the date, the person who found it and the quick one-line description are all important pieces of information for the summary reports. The detailed description is probably the most important field for the person trying to fix the bug. It is expected that the person writing the bug will do as much as possible to identify the conditions and circumstances under which the bug can be reproduced. If it can be reproduced, it can almost always be fixed.
Priority indicates how important it is that the bug be repaired.
Critical bugs are ones have some sort of workaround available, but need to be fixed quickly, because the workaround affects gameplay noticeably. Required bugs must be fixed before we ship, because they're significant enough to be noticed, but might not be urgent right now. Optional bugs are bugs that we don't consider critical to the game play, and that should be fixed if it doesn't increase the risk or affect the delivery date. Priority doesn't tell the whole story, though. We also have severity, which indicates how much of an impact the bug has upon further testing and upon the user of the game.