How and by whom is the electronic and physical infrastructure of the
Internet maintained? Individuals/organizations must maintain their own
sites; updating links, adding features and so on, but who and how
makes sure the whole system work together? Are they regulated by the
government? What incentives do they have for preventing interruptions
to the Internet? Is this a for-profit system?

The articles listed on this page provide much of the answer:
http://world-information.org/wio/infostructure/100437611791

Let us start by looking at the word "Internet".  "Inter" meaning
between.  "Net" is short for networks.  In a simplified view, the
Internet is literally the massive set of connections between various
networks.  Each connection is a link between two systems.

An example would be your computer which dials into a computer which is
owned by your Internet Service Provider (ISP).  Your ISP is then
connected to another network which provides the same service to them. 
In this example each entity along the way has motivation for
maintaining their point in the network because they are benefiting
from it.  You benefit by having access to information i.e. google.com.
 Your provider benefits because by providing a service to you they are
obtaining money.  Their provider obtains the same benefit from them.

There are entities which govern the Internet to various extents.  The
Internet Corporation for Assigned Names and Numbers (ICANN) manages
the domain name and the network number systems that allow you to type
in ://www.google.com instead of the series of numbers which are
the "virtual" numeric address for the Google computers.  ICANN has
been transitioning this control from the US government for several
years now.  ICANN is a not-for-profit corporation formed with the sole
purpose of managing the virtual addressing requirements of the
Internet.

Link:  Information about ICANN
http://www.icann.org/general/abouticann.htm

As with the physical linking, the creation and maintainace of web
sites are the sole responsibility of the owner of such sites.  Popular
web sites are normally well maintained to make sure all links are
valid.  "Dead" links or links which no longer reach the intended
target are quickly removed to prevent frustration from visitors.

All portions of the Internet from the physical to the virtual are
subject to the laws that are deemed to apply in any given area.  An
example is onling gambling.  Federal law prohibits gambling across
state lines.  As a result, you will not find any gambling web sites
which are physically located in the United States.  If a gambling web
site were to be hosted in Virginia and a user who dialed into the
Internet in Georgia accessed this site, the site would be in violation
of Federal law and subject to siezure and legal action.

As far as preventing interruptions, because the Internet is literally
a network of networks it is policed by the administrators of each of
the "nodes".  If someone using their connection to the network for
purposes against the contract they established, their link to the
Internet is subject to action from the administrators of the nearby
network nodes.

In summary, the Internet has become a largely for-profit system which
relies on self policing of a system maintained by a non-profit
organization.

jefros

The internet is not a single network.  It is made up of many small
networks and end users on those networks.  Look at it more like roads,
public and private.  Your driveway, owned by you crosses to a street
owned by the city.  From there you may travel county and state roads
on your journey.  You may or may not have access to certain private
roads. These roads connect to other driveways and parking lots.Some
roads, such as your driveway, are payed for directly by you.  Others,
such as city roads may be paid for by taxes.  Yet, other roads are
toll roads, payed for by those who use them.  The same is true of the
internet.

Chances are, you are using your own computer connected to your own
home wiring.  That in turn is connect via a telephone or cable line to
the telephone/cable company.  Depending on which ISP you have chosen,
you will take a certain route (virtual path)to reach the server of
your request.  Each section can be (and probably is) owned and
maintained by a different company.  As these companies do not provide
service for free, yes, it is for-profit.  How they make that profit
(subscribers, advertising, etc) can vary.

By the way, Al Gore did not invent it nor take the initiative in
creating it.