Managed: Varies a lot. Commonly it's only available with a control panel - the host will only support that control panel and its standard software. In some cases the host will do nothing unless you request it so you need to monitor the VPS yourself (ask about monitoring and proactive support).

If you know how to use linux and manage a server, go for unmanaged. If not, managed. With unmanaged they do nothing except make sure the network and hardware are up. Managed will actually keep the vps itself up and make sure it's all going smooth. But it's more expensive too.