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PVP is a plug-in that is available for free. You can find and enable PVP in the Optional Features list of the app.
PVP means Player Versus Player. So when you enable this plugin players will be able to fight each other everywhere in your server.
You can protect the spawn area of your server from players killing each other by using the plug-in Spawn Protection, so new players will not be discouraged from your server because of spawn-killing.
Use the plug-in Land Protection to prevent PVP in specific areas other than your server spawn.
Have a question about the plug-in PVP? Ask it on the forums here!