Towny is the dominating plugin on the server.
With it, you can create new towns which in turn protects land.
You can invite other players to these towns and build to your heart's content.
You can also create nations and invite towns to the nations.
What you do with your town and with your nation is up to you. There is no underlying thing theme, no specific objective. Build the best town, create the best nation, or simply just live in reserve in a hole.
Towny is a vast plugin, so this section helps to clarify and explain a few commands and concepts.