What is hdtv tuner?

Hi,
I have been doing some research on monitors and found some monitor has built in hd tuner(or digital tv tuner)
here are my questions about them…
1) do I need to have some kind of tv cable service (like cogeco, bell, or something)??
2) does it require PC to be connected in order to watch a tv?
3) is there a monthly fee that I need to worry about? (in other words, do i need to subscribe to some kind of service?)
4) how many channels are available?? or does it depends on my tv cable services if required?

I thought I know most of basic tech stuffs but this is pretty new field to me and it would be nice if someone can help me out