<h2>Summary</h2> Photoluminescence (PL) nanomaterials are widely used by scientists because of their important applications in optoelectronic devices and biomedicine, such as use in organic dyes, semiconductor quantum dots, perovskites quantum dots, fluorescent nanoclusters, and carbon dots (CDs). Among these, CDs have attracted widespread attention due to their outstanding PL properties. Therefore, a better understanding between CD fluorescence and afterglow emission is needed to achieve controllable structures and adjustable PL. In this review, we discuss the preparation and related applications of CDs by considering their structures and optical properties. First, the PL mechanism and the preparation methods of CDs with excellent optical properties are introduced. Subsequently, the adjustment of CD afterglow emission was highlighted from the perspective of intersystem crossing, triplet non-radiative decay rate, and the overall quenching rate. Then, we discuss the important optical applications of CDs. Finally, the research development and potential challenges of CDs are presented.