This chapter introduces the fundamental structure of phylogenetic trees, explaining various terms used to describe them, such as nodes, branches, roots, clades and sister taxa. It contrasts cladograms and phylograms, clarifying how branch lengths either lack meaning (cladogram) or represent evolutionary distance (phylogram). The concepts of rooted and unrooted trees are also introduced. Two methods of rooting trees, namely, midpoint and outgroup rooting, are discussed. The terms ‘monophyly’, ‘paraphyly’ and ‘polyphyly’ are defined, and their usage in describing the incongruence between traditional taxonomy and molecular phylogeny is explored. This foundation sets the stage for subsequent chapters.