When applications crash or are forced to quit during a hang in OS X, the system will bring up a crash reporter window that contains diagnostic information about your system, which applications were open, and memory traces for the threads that were open. This information allows developers to troubleshoot and fix bugs in their programs that may have led to the crash. When presented, you will have the option to send the report to Apple along with comments about what you were doing, or dismiss the message without sending.
After either sending or dismissing the report, you can find and manage them at a later date in order to help troubleshoot persistent problems, by using the Console application. Open this utility and click the button to show the Log list if it is not already present. Then locate and expand the following trees:
FILES --> ~/Library/Logs --> CrashReporter
FILES --> /Library/Logs --> DiagnosticReports
These Diagnostics reports will also be available in the "Diagnostic and Usage Information" section, where they will be listed under User and System sections.
In versions of OS X prior to 10.6.3, you are prompted with the option to send or dismiss this information whenever an error occurs, but in OS X 10.6.3 Apple has added the option to send this diagnostic information as well as usage data automatically. When you are first presented with a crash report after upgrading to 10.6.3, there will be an option to not be prompted again about sending the information to Apple. If you enable this option, the reports will be sent anonymously to Apple anytime you experience a system or application crash, hang, or other odd behavior.
In addition to sending diagnostics information to Apple, when you check this option the OS will also send non-diagnostic usage information about your computer and other applications to Apple. This information can be seen by selecting the "Diagnostic and Usage Messages" section in the Console application (it may take a while to populate thousands of messages), and show when an event occurred, what process was responsible for the event, and the type and result of the event along with an identifier string for Apple to organize the events.
Despite the anonymity of the reporting, many people may choose to not send this information to Apple automatically, but if you have already checked the box to do so, you can disable it by using the Console application's preferences, where you can uncheck the option to automatically send anonymous diagnostic and usage data to Apple. Keep in mind that when you deselect this option, the setting in the preferences will become inactive and you will not be able to enable it from the Console preferences anymore. It will only be activated again if you check the option to do so in the Crash Reporter window the next time you encounter a system error.