Circular RNAs (circRNAs) are a novel type of universal and diverse endogenous 
noncoding RNAs (ncRNAs) and they form a covalently closed continuous loop 
without 5' or 3' tails unlike linear RNAs. Most circRNAs are presented with 
characteristics of abundance, stability, conservatism, and often exhibiting 
tissue/developmental-stage-specific expression. CircRNAs are generated either 
from exons or introns by back splicing or lariat introns. CircRNAs play 
important roles as miRNA sponges, gene transcription and expression regulators, 
RNA-binding protein (RBP) sponges and protein/peptide translators. Emerging 
evidence revealed the function of circRNAs in cancer and may potentially serve 
as a required novel biomarker and therapeutic target for cancer treatment. In 
this review, we discuss about the origins, characteristics and functions of 
circRNA and how they work as miRNA sponges, gene transcription and expression 
regulators, RBP sponges in cancer as well as current research methods of 
circRNAs, providing evidence for the significance of circRNAs in cancer 
diagnosis and clinical treatment.