Current discussions of U.S. and global energy supply refer to oil, natural gas and coal using several terms that may be unfamiliar to some. The terms used to describe different types of fossil fuels have technically precise definitions, and misunderstanding or misuse of these terms may lead to errors and confusion in estimating energy available or making comparisons among fuels, regions or nations. This book describes the characteristics of fossil fuels that make it necessary to use precise terminology, summarises the major terms and their meanings, and provides a brief summary of the United States' endowment of fossil fuels and the relationship between the U.S. fossil fuel energy endowment and those of other nations.