Emacs Summary, the really, really short version

Notation
Starting Emacs from the command line

emacs start Emacs with an empty editing buffer
emacs filename
start Emacs and load the named file for editiing

Leaving Emacs

C-x C-c  End the Emacs session.
C-z  Temporarily suspend Emacs; use the command fg to return to it
This is not so important when you can open another terminal window.

Basics

C-h
start help mode
C-h t
start the Emacs tutorial (very useful, do this first)
C-g
cancel the current command
C-x u
undo

Moving the cursor
C-f  forward one character
C-b  back one character
M-f  forward one word
M-b  back one word
C-p  up one line (previous)
C-n
down one line (next)
C-a
start of line
C-e
end of line
M-a
start of sentence
M-e
end of sentence
C-v
forward one screen (also, PageDown)
M-v
back one screen (also, PageUp)
M-> forward to end of file
M-<
back to beginning of file

Files

C-x d
show the directory (press e to load a file)
C-x C-f
load a file
C-x s
save current file
C-x C-w  save current file under another name
C-x C-v  replace the current file with another
C-x i
inserts a file at the current cursor position

Editing
C-d  delete one character
C-k  delete (kill) to end of line
C-y  undelete (yank) last killed item
C-@  set mark
C-w  kill text between cursor and mark, use C-y to restore/move
M-w  same as above without deleting, use C-y to restore/copy
M-y
retrieve kill ring item, repeat to retrieve previously killed items
C-s
search forward for a string
C-r
search backward for a string
M-%
interactively replace a string