The files do not go anywhere, they stay on the computer. When a data file is created it uses a finite space on the hard drive and it causes some magnetic material on the hard drive to change state in the classic one's and zero's we have come to recognize as representation of computer data.
In order for the computer and its applications to recognize where data begins and ends, a particular series of one's and zero's are recorded before the data is recorded. This =header= code tells the computer and the associated application that the following codes are in fact data and can be read on screen or printed. There is an =ending= code to tell the computer where a data file ends.
When that data file is =trashed= the header code or =flag= is over written leaving the =data= or body of the file intact. Even though the data is intact it is not accessable by the computer for the application to be read or print.
If the file is long it will be =chopped= up into several segments and each segment will have a header code , not only telling the computer that data follows the header, but where the previous segment is located on the hard drive and where the following segment is located to. When too many files are broken up into segments the computer is required to hunt for them on the hard drive before loading them into memory for you to use. This hunting time does slow the computer down. When files are segmented the hard drive is said to be fragmented. A severely fragmented hard drive is a slow hard drive.
When you use a defragmenting utility the segmented files are brought back together and made into =contiguous= files again. The =extra= space once taken up by the fragmented segments are returned to the computer as =free space= meaning that there may be old data in the free space but it can be recorded over.
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