To copy data from a temporary area to a more permanent storage medium. When you edit a file with a word processor, for example, the word processor copies the entire file, or portions of the file, into an area of main memory called a buffer. Any changes you make to the file are made to the copy in the buffer, not to the real file on the disk. The buffer is temporary -- as soon as you exit the program or turn off the computer, the buffer disappears. To record your modifications to the file on the disk, you must save the file. When you do this, the word processor copies the contents of the buffer back to the file on the disk, replacing the previous version of the file.
Because computers can break down at any moment, it is a good idea to save your files periodically. Otherwise, you will lose all the work you have done during an editing session. Many applications automatically save files at regular intervals, which you can specify. These intermediate saves are sometimes called snapshots.