TRAMP (Transparent Remote Access, Multiple Protocols) is a package for editing remote files.

Open a file remotely with tramp under emacs

Inside emacs :

C-x C-f /remotehost:filename  RET (or /method:user@remotehost:filename)

Before launching emacs with ssh :

$ emacs /ssh:user@remotehost:filename

To speed up tramp, you shoud add this to your .emacs config.

;; tramp mode
;; Faster than the default scp
(setq tramp-default-method "ssh")