Dietary flavonoids represent a diverse group of plant-derived polyphenolic secondary metabolites that play an indispensable role in both plant physiology and human health. Characterized by a core 15-carbon skeleton structured as a C6-C3-C6 system, these compounds are universally distributed across vascular plants. This review synthesizes current research regarding the classification of flavonoids into distinct sub-categories specifically flavonols, flavan-3-ols, flavones, flavanonols, and anthocyanidins and highlights their primary dietary sources, which range from common fruits and vegetables to specialized functional beverages. Functioning as powerful antioxidants, flavonoids protect living cells from oxidative stress and subsequent macromolecular damage by rapidly scavenging free radicals and reactive oxygen species. Extensive in vitro data indicate that these molecules possess deep anti-allergic, anti-inflammatory, anti-microbial, and anti-carcinogenic properties. Furthermore, preliminary clinical studies demonstrate their therapeutic potential against cardiovascular diseases, chronic inflammation, and specific malignancies. However, significant challenges persist concerning their translational application, primarily driven by poor gastrointestinal absorption, rapid physiological metabolism, and rapid systemic excretion. This comprehensive analysis addresses the complex relationship between flavonoid structures and their biological activities, evaluates modern clinical breakthroughs, and provides strategic insights into metabolic engineering and functional food design to overcome current bioavailability barriers.