Circular RNAs (circRNAs) are a class of endogenous, covalently closed, single-stranded RNA without 3'-poly(A) and 5'-cap structures. CircRNAs are characterized by universality, diversity, stability and conservation, and have been found to regulate mammalian transcription and be translated into proteins. In this review, we summarized the biogenesis, classification, expression, distribution, biological functions and regulation of circRNAs. In addition, we discussed the association of circRNAs with diseases and the methods for identification and characterization of circRNAs. Finally, we speculated the application prospect and research direction of circRNAs.