Circular RNAs (circRNAs) are a class of single-stranded, non-coding RNAs which form a covalently closed loop. It was first reported as viroids in plants and was later detected in human cells in 1979. In a circular RNA molecule, unlike a linear RNA, the 5' and 3' ends have been joined together. The unique loop structure of circRNAs provides them with a longer half-life and more resistance to RNase R than linear RNAs. CircRNAs are produced by non-canonical splicing process which is known as "backsplicing". In the process of backsplicing, a downstream splice donor is joined to an upstream splice acceptor. According to the sequence composition of circRNAs, they are categorized into various types, such as exonic circRNA, exon-intron circRNA (EIciRNA), and circular intronic RNA (ciRNA) etc.