Towny capture sites allow you to battle on specific points for rewards for your town. Each capture site will allow you to earn rewards that can range from money or custom items that are not found anywhere else. Capture sites all have different rules, meaning some will let you keep your items and some will not so bear in mind. These are PvP regions that can have traps, unfair numbers in terms of battle, and other tactics.
A wrecked ship capture site
A battle site requires you and your town members to stand on it and hold it from the town that is currently holding it. If you successfully capture a site, the point becomes immune for 1 hour, and after that any town will be able to contest it. This allows you to earn rewards for the hour, but then have to defend it to continue earning them.
/t (sitename) - Shows you a status of the capture site with useful info
/t capture - Starts the capture of the site you are currently on. (Only the mayor or assistant can start a capture)
/t defend (sitename) - Teleports you to defend a capture site
/towny capturesites - Opens up a book with a ton of helpful information for your the capture sites.