To execute a command on a remote server:
ssh -t user@example.com 'the-remote-command'

To tunnel an x session over SSH:
ssh -X user@example.com

To launch a specific x application over SSH:
ssh -X -t user@example.com 'chromium-browser'

To ssh via pem file (normally needs 0600 permissions):
ssh -i /path/to/file.pem user@example.com

For more information, see:
http://unix.stackexchange.com/q/12755/44856