Alternative energy is a very general term referring to nontraditional sources of electricity. Those traditional sources are out of favor with some political groups because they use exhaustible fuels, they pollute, or both.
Ideal alternative energy sources cause little or no pollution. Traditional sources require mining or other environmental disruption to acquire, and create noxious fumes, toxic waste, and other side effects in use. The division is not always clear. Nuclear power from uranium, plutonium, or even thorium causes little or no traditional pollution, but is typically opposed by environmental factions because of its own unique problem with waste disposal.
In the end, alternative energy is usually "alternative" because it is less economical or less well-developed than traditional sources. Most electricity is, after all, produced by commercial interests or at least by public utilities with some budget limitations. For these decision-makers, nontraditional choices of technology usually require some kind of government mandate or subsidy to be attractive.
Department of Energy figures for 2014 show that about 40% of US electricity on the power grid was generated from coal. Hydroelectric was the largest renewable source at 6% of the total. Renewable sources collectively supplied 13% of the total, with wind being the second-largest.