A tablet without 3G will only work with Wi-Fi, so you have to be near a wireless router. Many homes now have a wireless router but also many shops, pubs, restaurants also have wi fi.
I would guess most tablets people buy are wi fi only.
But some tablets come with wi fi AND 3G.
So they do wi fi like all tablets.
But the ones with 3G can also act like a mobile phone, you put a sim in it and you can connect to the internet anywhere, even if there is no wi fi around.
However you will have to pay for the sim (monthly charge) so that will be an on going cost.
If you buy the wi fi only device it will have no on going cost.