So an application was very rude to you and had a hard crash like this one here:
If you click on OK the report will be sent to Apple. But how do you get the data back if you closed the window and you need the data for some reason?
First unhide the Library folder. Now go to Library -> Application Support -> Logs -> Diagnostic Reports like shown here:
The files have the names of the applications they belong to. In the screenshot I can see that I have a problem with a helper app from Evernote which crashes a couple of times a day without me noticing the crash.
What is really funny is that I had to do the screenshots on my MacBook Air. My iMac has a small problem: there are no crash logs - at all. The app just crashes. There is no „Do you want to send the log to Apple?“ window. No crash logs in the above folder. Nothing. The problem is so rare that even the Goggle wasn’t able to tell me anything about it. But I can always test on my MacBook Air. The problem isn’t worth the hassle of a reinstallation.