ssh (secure shell) is a program that lets you work with remote unix boxes.
Use keypair authentication whenever possible.
Generate a keypair with ssh-keygen -t rsa. Copy your public key to a server with ssh-copy-id -i ~/.ssh/id_rsa.pub adm@server.org
System-wide at /etc/ssh/ssh_config and per user at ~/.ssh/config
To quickly connect to a system using its domain name instead, add an entry for your host in the config:
Host server server.coolnet.org
Hostname 221.140.72.18
User opc
This way you can ssh server and you login as the opc user.
To specify a keyfile use IdentityFile ~/.ssh/key. The IdentitiesOnly yes option forces the use of the specific keyfile.
SSH Tunnels can be used to access a local port from a remote machine.
For example, if you have Syncthing's Web GUI running on port 8384 you can forward it to port 9384 on your local machine to access it securely.
To open a SSH tunnel use ssh -L [LOCAL_IP:]LOCAL_PORT:DESTINATION:DESTINATION_PORT [USER@]SSH_SERVER
For example ssh -L 9384:localhost:8384 debian@orion
TCP Forwarding can be specified in a config file with the LocalForward option.
SSH Agents let you cache your private keys temporarily after first use. This way you don't need to enter the same passwords over and over again.
By default, your ssh config doesn't use any agent. To make use of openSSH's built in one (ssh-agent), include AddKeysToAgent yes at the top of your config.