The ASD extension is used by Microsoft Word version 95 or later for auto-save files. Word stores the edited document independently at regular, adjustable intervals. Files with the extension ASD are created in the temporary directory. These files are deleted when the document is saved properly.
However, if Word did not correctly save a document due to an error or crash, this backup file can be used to restore the document. This usually happens automatically. It can also happen that this does not happen, then you can search for the ASD file and load it with Word (show all files).