1. In general, close is the action performed to terminate a portion of a program or the complete program. If a file has changed since it was opened and the program is closed, any changes done to the open file may be lost. If the program closes without performing the close action, this is usually referred to as a crash.
2. When referring to a button or a feature in a GUI operating system, the close box or close button allows a user to terminate a window within a program or the complete program. For example, In Microsoft Windows operating systems and many other operating systems and software programs, the close button with a "X" is located in the top right corner of the window. If there are multiple windows opened within the program, the "X" at the top right corner will close the program and the "X" beneath will close the window, but keep the program open. In the below picture, is an example of the close button.
Tip: In Apple Mac OS operating system, the close button is represented as red button in the top-right corner of the window.
3. Close is also a command used to exit a program. For example, in a FTP session close can be used to exit the session. See the how to use ftp help page for information about close and other FTP commands.