Basically Towny is a 'peaceful' version of factions. The general idea of a group of players forming and building on a piece of land still applies, with these exceptions:
1)There is no member limit
2)You can create 'outposts', which can be anywhere
3)Land does not have to be connected
4)No such thing as raiding </3
Overall it, opposed to factions, focuses on buliding, not pvp, and therefore appeals to me.