Bacteriophages (phages or bacterial viruses) are the most abundant biological entities in our planet; their influence reaches far beyond the microorganisms they parasitize. Phages are present in every environment and shape up every bacterial population in both active and passive ways. They participate in the circulation of organic matter and drive the evolution of microorganisms by horizontal gene transfer at unprecedented scales. The mass flow of genetic information in the microbial world influences the biosphere and poses challenges for science and medicine. The genetic flow, however, depends on the fate of the viral DNA injected into the bacterial cell. The archetypal notion of phages only engaging in predator-prey relationships is slowly fading. Because of their varied development cycles, environmental conditions, and the diversity of microorganisms they parasitize, phages form a dense and highly complex web of dependencies, which has important consequences for life on Earth. The sophisticated phage-bacteria interplay includes both aggressive action (bacterial lysis) and "diplomatic negotiations (prophage domestication). Here, we review the most important mechanisms of interactions between phages and bacteria and their evolutionary consequences influencing their biodiversity.