Severities

error

used when bugs are found

warning

suggestions about defensive programming to prevent bugs

style

stylistic issues related to code cleanup (unused functions, redundant code, constness, and such)

performance

Suggestions for making the code faster. These suggestions are only based on common knowledge. It is not certain you'll get any measurable difference in speed by fixing these messages.

portability

portability warnings. 64-bit portability. code might work different on different compilers. etc.

information

Configuration problems. The recommendation is to only enable these during configuration.