You have two options for Internet connectivity - wifi through your home network and other locations (like Starbucks, hotels, libraries, etc) or cellular service through ATT or Verizon. All models come with wifi; cellular is optional for $ 130 extra. If you get the cellular option, you pick one or the other and are stuck with it - the cell modems are carrier specific. And, of course, cell service is not free.
Browse the forums - lots of threads pro and con about getting cell service and which provider to select. Me - I carry my IPAD everywhere, so I went with the ATT option. Again, lots of spirited debate out there about cell carriers.
BTW - if don't have Internet service at your home, you really need to get it along with a wireless router.
Hope this helps