When we talk about alternative fuel, we basically mean energy sources other than fossil fuels, in particular energy sources that are readily renewable, like wind, solar, and geothermal power. Dependence on fossil fuels has two major problems. One is the fact that we are extracting fossil fuels from the planet much faster than they are being replenished. The other is the fact that burning fossil fuels produces some level of pollution. And even the cleanest burning fuel will still produce carbon dioxide, a greenhouse gas that could have a measurable impact on planetary weather cycles.
The unfortunate reality is that there really is no chance that human civilization can exist without any use of fossil fuels anytime in the foreseeable future. One reason is that there is a global infrustructure in place for fossil fuels, and an entrenched infrastructure is very, very difficult to eliminate or make major changes to. Another reason is that fire is both portable, universal, and releases an enormous amount of energy in the form of heat from a relatively small flame. Internal combustion engines are not particularly efficient at converting the energy of the fuel into motion of a car (20% at best), but combustion produces so much energy and for decades fuel was so cheap and the consequences unknown that it didn't really matter. And we really need the energy of combustion to operate larger, portable machinery like construction equipment. So we must accept from the outset that some level of fossil fuel burning is simply unavoidable for the foreseeable future.
The reality here is that is no such thing as a magic bullet when it comes to energy. In other words, there is not a single, magic solution that will take care of everything. The truth is that a mix of energy sources will be required in the future. Fossil fuels in combination with wind, solar, and geothermal will be used where appropriate and as circumstances dictate. There is even a place for nuclear power even though it tends to get a bad reputation in the wake of an accident. Solar will not really be realistic in parts of the world with high precipitation, but more arid areas are prime targets for a higher percentage of solar power in the mix. Coastal areas could really benefit from utilizing wave power in the oceans for powering their grids which are usually population dense areas.
In specific terms of transportation, I think that natural gas is an excellent alternative to traditional gasoline. It is cleaner burning than gas and more efficient. It still produces carbon dioxide, but it makes sense at least as a transitional fuel to try and move passenger vehicles away from gasoline. Natural gas could be a real benefit while electric vehicles and battery technology are being continually improved to the point that they are ready to take over as the main fuel for passenger vehicles.