OpenSSH is the tool of choice to use for remote shell access, remote file transfers, remote command execution, and the tunneling of network connections for security. It has many different capabilities.
For remote commands and file transfers, start with the man pages for the client tools: ssh(1), sftp(1), scp(1). See the man page for the server daemon sshd(8), and you will also want to review the man pages for the configuration files, config_sshd(5) and config_ssh(5).
Michael W. Lucas has just published a book,
SSH Mastery. See
www.openbsd.org/books.html for more information.