Circular RNAs (circRNAs) are a unique class of endogenous RNAs that are characterized by their covalently closed loop structures, with no 5′ end cap or 3′ polyadenylated tail. They are formed through a process called reverse splicing, where the downstream splice donor of precursor mRNA (pre-mRNA) is joined to an upstream splice acceptor. This process can occur across numerous genes within eukaryotic organisms, giving rise to a diverse array of circRNA molecules. Although circRNAs are generally expressed at lower levels compared to linear RNAs, recent studies have shed light on their expression patterns, which are notably cell type-specific and tissue-specific. This specificity suggests that circRNAs play crucial, context-dependent roles within the biological processes of different cells and tissues. Further investigations into the regulatory mechanisms of circRNA biogenesis have revealed a complex network of influences that govern their formation. These mechanisms ensure that circRNAs are produced in a controlled manner, contributing to their functional roles in the cells. One of the most intriguing aspects of circRNA function is their capacity to influence gene expression. CircRNAs can act as molecular sponges for miRNAs, a process by which they bind and sequester these small, non-coding RNAs, thereby modulating their ability to regulate target mRNAs. This interaction can have profound effects on gene expression patterns and cellular function. In addition to their role in microRNA sequestration, circRNAs can also affect gene expression directly by interacting with transcription machinery or indirectly by interfering with splicing processes. By disrupting or modifying splicing, circRNAs can alter the splicing patterns of other pre-mRNAs, leading to diverse outcomes in gene expression profiles. These multifaceted roles of circRNAs add a significant layer of complexity and diversity to the transcriptomes of eukaryotic organisms, indicating that circRNAs are more than mere by-products of splicing—they are active participants in the regulation of gene expression and have potential implications in development, differentiation, and disease. The ongoing exploration of circRNAs promises to further unravel their functions and mechanisms, potentially opening new avenues for therapeutic intervention.