In computing, overwriting refers to replacing old data with new data. There are two primary types of overwriting: 1) replacing text, and 2) replacing files.
1) Replacing text
The default behavior of most word processing programs is to insert characters where the cursor is located. However, some programs allow you to change the standard behavior from insert to overwrite (or "overtype"). If an application supports both modes, the Insert (INS) key can often be used to toggle between insert and overwrite mode.
While in insert mode, text to the right of the cursor is shifted to the right as new text is entered. For example, say you want to add the word "and" between "five" and "six," in the string "four, five, six." You would move the cursor immediately before the word "six," then type "and" (followed by a space). The result would be "four, five, and six." In overwrite mode, the word "six" would be overwritten by the word "and," so the resulting string would read, "four, five, and." Overwrite mode simply replaces existing characters as you type.
2) Replacing files
The term "overwrite" also refers to replacing old files with new ones. If you try to save a document with the same filename as an existing document, you may be asked if you want to overwrite the file. If you click OK, the old document will be overwritten by the new one. Similarly, when moving files to a folder, the operating system may ask you if you would like to overwrite existing files with the same filenames. If you choose select Overwrite, the old files will be replaced by the new ones.