Natural resources are comprised of bonus resources, strategic resources, and luxury resources. Luxury resources are goods that improve the happiness of your cities. Strategic resources are needed to make certain military units (iron, for example, is needed to make the Swordsman or Roman Legion units) Bonus resources are resources like gold that simply produce extra food, commerce, or shields.
Yes. Though it happens infrequently, resources will sometimes be exhausted, forcing you to find a new source.
Many resources will not be visible on the world map until you have unlocked the secrets of a related technology. You will not see iron on the map, for example, until your scientists have discovered Iron Working. Likewise, uranium will not be visible until you have discovered Fission. The game will distribute resources throughout the map so that each Civ will have access to several nearby resources. Additionally, simply by allocating population points to work the tiles inside your city radius, there is a chance each turn that your citizens will discover a new source of a known resource.
To access any good, you need to
build a road to that good. That good must also be connected to your capitol in some way, be it by road, harbor, or airport. If the good in question lies outside of your borders, you will also need to build a colony on that square. For luxury resources, all cities connected to the trade network will automatically receive the benefit of the luxury. For resources, all networked cities should now be able to build units that require that resource.
You can trade goods with another civ as long as you have a road, harbor, or airport that connects your civilization with theirs. Once you have a trade route, you simply negotiate with the other civilization in the diplomacy screen.
No, trade has been abstracted to the diplomacy and trade advisors and will no longer require you to use caravan units.
There are two ways to establish a trade route between two cities on different continents, or in situations where roads or railroads cannot link two cities in any way: airports and harbors.
Using airports, both cities need only have airports to enable trade.
To trade via harbors, both cities need not only have harbors, but there must also be an unblocked path of navigable tiles between the two cities. This means that if your current research level (i.e., before Astronomy) restricts your naval vessels from leaving coast tiles, only coast-based trade routes can be established. Finally, rival vessels can block this route, preventing naval trade between two cities.