Its a visual editor by default installed in every linux system.

The next version of vi is vim.

The vi editor tool is an interactive tool as it displays changes made in the file on the screen while you edit the file.
Modes: It has two modes.

Command Mode: Initially vi is in command mode.To pass any command it should be in command mode.

Insert Mode: In this mode,entered text will be inserted in to the file. 

Esc key is used to change from insert mode to command. To enter into insert mode press i now we can type whatever we want.

Note:

1.If we are not sure we are in which mode then press Esc  key twice then automatically will be in command mode.

2. Its case sensitive cmd: vi file_name