Anything is a pretty wide ranging thing to ask about.
To take your example of emails, it depends on the email system/software you are using, but the most common way it's dealt with would be they are moved to a deleted items folder which may (or may not) be automatically cleared after either a set period of time, or when the amount/size of the mails in the deleted items exceeds a certain level.
Anything that is deleted or uninstalled on your PC is still there on the hard drive.
All windows does is to deny access to these files. They stay there until you start filing up your hard drive with other programmes and downloads, which eventually get placed over the top of the deleted files.