If the computer gives a wrong answer (verify that the answer is wrong!) or crashes, it is a bug. If any one program crashes or gives an operating system error message, it is a bug.
If a command never finishes running can be a bug, but you must make sure that you didn't tell it to take a long time doing whatever you wanted it to do. Ask for assistance if you didn't know what the command did.
Some messages will alert you of bugs. Some messages are not bugs. Check Section and any other documentation to make sure they aren't normal informational messages. For instance, messages like ``disk full'' or ``lp0 on fire'' aren't software problems, but something wrong with your hardware--not enough disk space, or a bad printer.
If you can't find anything about a program, it is a bug in the documentation, and you should contact the author of that program and offer to write it yourself. If something is incorrect in existing documentation, it is a bug with that manual. If something appears incomplete or unclear in the manual, that is a bug.
If you can't beat gnuchess at chess, it is a flaw with your chess algorithm, but not necessarily a bug with your brain.