Are files ever deleted from our HDD

When a file is sent to the recycle bin, all that really happens is that the index to that file is moved to a special (reserved) folder, which is what the Recycle Bin really is. The file itself - that is, the data stored on the disk, is not moved or deleted at all. When you 'empty the Recycle Bin', all that happens is that the index entries to the files are removed. Even these index entries are not actually deleted. They are renamed so that they are no longer active. When a file is deleted, then, all that happens is that the operating system 'sees' the space where the file itself still is as 'free space' ... a place where new data may be stored when it needs to do so. The data still exists and the file can easily be completely recovered at that point. As soon as new files are created, however, the operating system looks for free space to store them and it may begin to use the space where the deleted files are, overwriting the old data with new data and making an index to that space for the new file. What data recovery programs do is to look at ALL the index entries, including the ones that have been disabled by being renamed. They then look at the physical locations which these old index entries point to, compare them with the other indexes and, if there is no match, they know that the old data still exists and can be recovered by resetting the old index to be active again. Sometimes part of the data will have been overwritten by new files and the file recovery software may then not be able to recover the file. That is why it is essential, if you know you have accidentally deleted files, to do NOTHING more on the computer before you run the file recovery software.