VI modes

vi has two different modes. One is Command Mode, the other is Text Entry Mode:

• Command Mode
  In Command Mode everything that you type is interpreted as a command. For most commands, a single keystroke is the entire command, while other commands require more information, such as a linecount followed by a single-character command. Command mode is used to perform powerful editing functions, as well as such actions as writing the text buffer to a file. To enter Command Mode from Text Entry mode, type an <ESC> character.
• Text Entry Mode
  In Text Entry mode all characters typed are printed on the screen and become part of the current document. Editing in this mode is restricted to insertion and typeover. To enter Text Entry Mode from Command Mode, type the character 'i' (for insert mode), 'a' (for append mode), or 'o' (for open mode). The significance of the different modes will be discussed later in the section 'Using VI'.