vi Commands 

i - Insert at cursor (goes into insert mode)
a - Write after cursor (goes into insert mode)
A - Write at the end of line (goes into insert mode)
ESC - Terminate insert mode
u - Undo last change
U - Undo all changes to the entire line
o - Open a new line (goes into insert mode)
dd - Delete line
3dd - Delete 3 lines.
D - Delete contents of line after the cursor
C - Delete contents of a line after the cursor and insert new text. Press ESC key to end insertion.
dw - Delete word
4dw - Delete 4 words
cw - Change word
x - Delete character at the cursor
r - Replace character
R - Overwrite characters from cursor onward
s - Substitute one character under cursor continue to insert
S - Substitute entire line and begin to insert at the beginning of the line
~ - Change case of individual character

Moving within a file

k - Move cursor up
j - Move cursor down
h - Move cursor left
l - Move cursor right

Saving and Closing the file

Shift+zz - Save the file and quit
:w - Save the file but keep it open
:q - Quit without saving
:wq - Save the file and quit