It connects via an hdmi port and pulls its content via an internet connection (wifi or ethernet to your router). I can't see any reason that you can't run other boxes from other providers at the same time as long as you have spare hdmi ports on your tv. You then just switch inputs on your tv (hdmi 1, hdmi 2 etc.) to view the appropriate box / content.
With a NowTV box you will need a spare HDMI input port on your television and an internet connection speed of at least 2.5Mbps where you will need to connect the box over either WiFi or Ethernet to your Router.
To use the NowTV service (apart from third party Apps) you will need to buy NowTV Passes and can pick and match between Cinema, Entertainment, Sports, Kids or Hayu.
You won't be able to watch over the air Freeview channels from the NowTV box (apart from the older discontinued Freeview box) but what i do is use my BT Youview box for watching live or recording Freeview channels.
NowTV will be adding the Entertainment, Sports, Kids & Hayu passes to their NowTV App in the near future on the Youview Box platform (though i am not sure what Youview boxes will receive it).
I have seen a sneak preview of the updated NowTV App which looked pretty good for the short time that i had a chance to play around with it (though it is still Work in Progress from a couple of bugs that i noticed) where the beta App was mistakenly sent to my BT Youview box in error.