These are functions dealing with error handling and logging. They allow you to define your own error handling rules, as well as modify the way the errors can be logged. This allows you to change and enhance error reporting to suit your needs.
With the logging functions, you can send messages directly to other machines, to an email (or email to pager gateway!), to system logs, etc., so you can selectively log and monitor the most important parts of your applications and websites.
The error reporting functions allow you to customize what level and kind of error feedback is given, ranging from simple notices to customized functions returned during errors.