With the increasing global emphasis on renewable energy, the generation of biomass fly ash is anticipated to rise substantially in the coming years. This trend highlights the urgent need for effective strategies to manage and utilize this byproduct sustainably. This study reviews and classifies selected types of biomass ashes, treating them as representative examples that illustrate the wide variability in chemical compositions and physical properties across different biomass sources. These include ashes derived from woody biomass, agricultural residues, animal-origin biomass, and sewage sludge. Biomass ashes exhibit significant heterogeneity compared to coal ashes, which makes understanding their chemical composition—particularly the major components such as calcium (Ca), silicon (Si), and phosphorus (P)—critical for identifying suitable industrial applications. Special categories of ashes with elevated heavy metals and chlorine (Cl) levels are also identified and discussed. This approach enables the identification of sustainable utilization pathways, ranging from traditional uses, such as agricultural fertilizers and construction materials, to emerging applications in advanced engineering materials. Additionally, this paper addresses the assessment and mitigation of potential risks related to hazardous metals in biomass ashes.