Well I can describe my hive. Whether the latest ones are like this I don't know.
There are three boxes.
i) Controller/Relay : This is a white box in the airing cupboard. It is connected to the mains, the boiler and the valves that switch between ch/dhw and both at once. it connects to the hub via zigbee.
ii) Hub : A white box that connects to the router via a LAN cable. This communicates to the thermostat, the controller/relay via zigbee and allows control of ch/dhw over the internet via the app.
iii) Thermostat : A white box with a big dial on the front. This allows you to control the temperature of the ch, start dhw generation, change timings etc. You don't really need this, as most of the stuff is better done via the app.
If you are missing the controller/relay or the Hub it won't work, so you need to have these. If it is an old system I guess you will have to reset it. The hive website should describe this. If it is a new system you can set up the accounts fairly easily and install the hub. The controller/relay needs a bit more work as you need to wire it into the mains, so you need to at least have an idea of what you are doing.
As to whether it is worth it - I think yes. It allows you to control CH while you are out, and often to a much better resolution than you can on a normal thermostat and to more erratic patterns (for example come on at 17:00 every day apart from Tuesday when I am out until 21:00). You can get the house heated better and save money at the same time.
Is it the best system out there ? Hmmm. Depends what you want I guess. Nest is probably better if you want to config different stuff and add it in, and generally fiddle with stuff. Hive is better if you want a smart looking integrated solution. You can buy add on bits for Hive, but they tend to be more expensive.
If you know phones, I would say Hive is kind of similar to Apple. Whereas Nest is more like Android. Less polished, but easier to do weird stuff and integrate different devices.