The ex line editor serves as the foundation for the screen editor vi. Commands in ex work on the current line or on a range of lines in a file. Most often, you use ex from within vi. In vi, ex commands are preceded by a colon and entered by pressing Enter.
You can also invoke ex on its own--from the command line--just as you would invoke vi. (You could execute an ex script this way.) You can also use the vi command Q to quit the vi editor and enter ex.