This book provides a basic and essential introduction to ecology. It was prepared and designed to enable students of applied biology and other related disciplines to gain enough knowledge of the main branches of ecology and the relationships between them. It is also important to those professionals and researchers carrying out field works and studies requiring the knowledge of ecosystem and its various factors. An introduction to this book gives basic ecological definitions which are often mistakenly considered to be known and which are important for an adequate understanding of ecology. Emphasis is given to clarity and readability to make easier the understanding of the following core concepts of ecology: practical ecology, biosphere, wildlife, ecosystem, trophic levels, biochemical cycles, population, community and interactions between organisms.