SSH provides secure communication across a network. It encrypts all traffic in both directions to make it secure. When you use SSH to log onto a remote machine, the password you will be prompted should be safe as should any other sensitive information you exchange. SCP is a service that runs over SSH which allows files to be transferred to/from a remote machine. SSH provides an authorisation service that can allow you to login to a remote machine, or exchange files, without having to supply a password for the account you access on the remote machine.