ELMAH stands for Error Logging Modules And Handlers for ASP.NET.

Wondering in Scott Hanselman blog i know about this one. Scott Hanselman ELMAH

The tool helps us monitoring the exceptions have been happening during the life of web application. Why do you need that? Stack trace and its suffs are useful information in order to fix bug. With this tool, you do not need to modify, or recompile your application. Just a few steps with web.config then you get it in hand.

You might also want to read about rule of exception handling  here