Some time while you are testing you need to debug some things in your asp.net application. Suppose you have run the application with ctrl + f5 and now you want to debug some things, then there are two things either you close the browser and press f5 and start debuging but if your application is heavy that will waste lots of time for your application, second way is some thing tricky but you can debug things.
We have aspnet_wp.exe(asp.net worker process) running on machine where iis and application is installed. Now suppose at present you are browsing default.aspx page and you want to debug it without pressing f5 . Then go to the your visual studio IDE -> Debug menu-> Attach Process. Select the aspnet_wp.exe process and click 'attach to process'. Now your application instance will bind with code. You can set the debug point as you do with normal debugging and it will work just as normal debugging.
So you can start debugging without wasting so much time.