is Command Line Interface to install salesforce packages that is managed as remote repository.
It's simpler than officially supported packaging system.
Because SPM is command line interface tool, you can combinate other *nix commands.
SPM can also be used as deployment tool. It can install from an salesforce organization to other.
"Deploy to Salesforce Button
" is similar to SPM.
But these tool is different in interface, CUI (spm) or GUI (Deploy to Salesforce).