This article is currently lacking one or more significant sections; these sections may need further research or thought and are expected to be completed later as time permits.
The United States in particular, and the global economy in general, depends heavily on non-renewable fossil oil as a fuel for transportation. More specifically, it depends upon oil remaining relatively inexpensive.
In practical terms, a "fuel" is any substance which can hold enough energy to power a reasonably-sized vehicle for a distance of at least several hundred miles while fitting into a container small enough to be carried by that same vehicle. The key fact here is not that the fuel provides the energy, but that it contains it. An extension cord plugged into a household power socket can provide a lot of energy, but you can't take it very far. Conversely, most batteries (anything from a miniscule Lithium watch battery to an automotive wet-cell battery) are quite portable, but cannot hold enough power to move a vehicle any useful distance.
The most important thing about oil, which is even more true once it is refined into chemicals such as gasoline (petrolium) and diesel, is that it has a high energy-to-volume ratio – in other words, you can pack a lot of energy into a relatively small space. There are very few substances which approach this density, and most of them create other problems -- hydrogen, for example, is far more combustible than gasoline, and is extremely hazardous to transport in quantities large enough to be useful as a fuel.
To the best of our knowledge, naturally-occurring oil is formed by the actions of heated chemicals under extreme pressure over "geological time scales" (millions of years), and therefore is not something of which we can quickly make more. Once the naturally-occurring supplies have been exhausted, we will need to have found either an alternate method of production or a better means of storing energy.
(to be written)