Circular RNA (circRNA) is a type of RNA molecule characterized by a covalently closed loop structure, in contrast to the linear structure of most RNA molecules. CircRNAs are produced through a process called back-splicing, where a downstream splice acceptor site joins with an upstream splice donor site to form a circular molecule. CircRNAs have been found to be highly abundant in various organisms, including humans, and are emerging as important regulators of gene expression. They have been shown to play roles in a variety of cellular processes, such as acting as microRNA sponges, regulating gene transcription, and interacting with RNA-binding proteins. Research on circRNAs is gaining increasing attention due to their potential implications in various diseases, including cancer, neurodegenerative disorders, and cardiovascular diseases. Understanding the functions and mechanisms of circRNAs may lead to new insights into disease pathogenesis and potential therapeutic targets.