Research on the microbiome has progressed from identifying specific microbial communities to exploring how these organisms produce and modify metabolites that impact a wide range of health conditions, including gastrointestinal, metabolic, autoimmune, and neurodegenerative diseases. This review provides an overview of the bacteria commonly found in the intestinal tract, focusing on their main functional outputs. We explore biomarkers that not only indicate a well-balanced microbiota but also potential dysbiosis, which could foreshadow susceptibility to future health conditions. Additionally, it discusses the establishment of the microbiota during the early years of life, examining factors such as gestational age at birth, type of delivery, antibiotic intake, and genetic and environmental influences. Through a comprehensive analysis of current research, this article aims to enhance our understanding of the microbiota’s foundational development and its long-term implications for health and disease management.