Temporary files
From Just Solve the File Format Problem
Temporary files are used by applications for different purposes, such as saving state or caching. Unix-style operating systems generally have a /tmp/ directory to hold temporary files, and Windows uses a Temp/ directory beneath the Windows directory. However, temporary files might be created in other places as well.
Programs might use files with a .bak extension to store temporary backup copies of the previous version of a file that has been modified (which might be of any format), and files with the .tmp extension, as well as various extensions with special characters such as $ and _ which rarely occur in file extensions otherwise, might be used for temporary work files of various sorts.
