Secure Shell Daemon allows remote machines to securely log in to the current machine. Remote machines can execute commands as it is executed at this machine.
# To start daemon in the background:
sshd
# To run sshd in the foreground:
sshd -D
# To run with verbose output (for debugging):
sshd -D -d
# To run on a specific port:
sshd -p <port>