Smart Subscriptions is distributed as a package installable via Joomla Extensions manager. The core package includes the component itself, "Smart Subscriptions" user profile plugin and "Offline" payment plugin, all these extentions are installed automatically. The built-in plugins are not enabled by default, so that your site behavior will not change until you enable them manually. You can start adding accounts to Smart Subscriptions right away after installation. Normally you will also have to prepare user groups / view levels for your planned subscriptions (see Configure user groups for Smart Subscription accounts for details).
Optionally, download and install payment plugins, "demo" plugin is available as free download. You will have to enable these plugins manually after installation. Once you have accounts and user groups basically set up you can enable "Smart Subscriptions" user plugin - at this moment your site members and visitors will see subscriptions selector in registration and profile forms.
Payment plugins and other add-ons for Smart Subscriptions can be found here.
Smart Subscriptions component can be uninstalled via Joomla Extension manager. The user plugin and all additional payment plugins will be uninstalled automatically along with the main component. Uninstallation script will also clean up user profiles table removing all related keys. It should not affect any other component or plugin.