Formats of various OpenSSH config files:

~/.ssh/config

See ssh_config(5).

~/.ssh/known_hosts

See the section "SSH_KNOWN_HOSTS FILE FORMAT" of sshd(8).

~/.ssh/authorized_keys

See the section "AUTHORIZED_KEYS FILE FORMAT" of sshd(8).

The command= option allows for some very interesting uses. For instance, see how Gitolite uses it. Gitolite effectively creates a "whole new set of users+permissions, nested within (but separate from) the Unix users+permissions."