On Towny Minecraft servers players can create Towns, Towny plugin then allows Towns to claim land, buy and sell plots and run Town politics.
About Towny servers...
Towny servers are generally like Survival Minecraft servers but with different kind of claiming plugin called Towny, each Town has own Mayor who is in charge of that Town, which includes managing the residents and setting the Town rules.
Economy on Towny servers?
All Towny servers also have full ingame economy, which includes Town banks, the ability to purchase Town land with ingame currency and a taxation system for Town residents.
On Towny servers players can work together and against each other as they see fit, usually large Towns would compete with each other.