Simple phenolic acids are considered the most common and widely studied bioactive compounds belonging to plants’ secondary metabolites. The simple phenolic acids can be classified as hydroxybenzoic (gallic, protocatechuic, p-hydroxybenzoic, and syringic acids) and hydroxycinnamic acids (p-coumaric, caffeic, ferulic, and sinapic acids) and present high commercial value. This review comprehensively discussed the trends, classification, sources, biosynthesis, and biological properties of hydroxybenzoic and hydroxycinnamic acids, focusing on the antioxidant, anticancer, antitumor, anti-diabetic, anti-inflammatory, antimicrobial, anticholesterolemic, antimutagenic, and antihypertensive activities. The findings obtained support the application of simple phenolic acids in the cosmetic, food, pharmaceutical, and health industries and provide future perspectives and guidance for the exploration of simple phenolic acids from plants and fruit sources.