Plugins are Sputnik-specific modules that add functionality. They can be installed using LuaRocks, just like any other rocks. There are two types of plugins: "options" and "applications".
Options plugins provide alternatives to existing Sputnik functionality. See:
Applications plugins essentially install new applications based on Sputnik. For now, the only one we've got is "sputnik-tickets" which installes a bug tracker. After installing Sputnik (see Installation), you can install Tickets with:
luarocks --only-from=http://sputnik.freewisdom.org/rocks/earth install sputnik-tickets
Then go to the page called "Tickets". Note that this page didn't exist before you installed the plugin, and now it does. If you click on "Create New Ticket", you will be taken to an edit form for a new page ("Ticket:000001"), which has it's prototype pre-set to "@Ticket" and because of this has a custom edit form. Note that "@Ticket" is again a new page, which got installed by the plugin.
More specifically, what the plugin does is add four new modules:
sputnik/actions/tickets.lua sputnik/node_defaults/@Ticket.lua sputnik/node_defaults/Tickets.lua sputnik/node_defaults/_translations_for_ticket.lua.lua
The first is a real module that provides a new "show" action for the "Tickets" page. The other three modules just hold the data for the new pages. The actual Versium pages will be created on the fly when they are accessed.