The complex world of RNA molecules is explored with a particular emphasis on noncoding RNAs (ncRNAs) and the variety of functions they play in posttranscriptional gene regulation. This chapter covers the classification and traits of ncRNAs in eukaryotic genomes. The diverse functions of ncRNAs are highlighted, including their involvement in posttranscriptional processes such mRNA splicing, degradation, and translation, as well as transcriptional and epigenetic control. The relationships between long noncoding RNAs (lncRNAs) and microRNAs (miRNAs) are further explored in this chapter, emphasizing the function of lncRNAs as competitive endogenous RNAs (ceRNAs) and control expression of gene. The chapter also covers how lncRNAs are involved in splicing and chromatin remodeling. In conclusion, the chapter delves into the newly discovered function of circular RNAs (circRNAs) in the regulation of alternative splicing, highlighting their influence on the complexity of plant genomes and the regulation of genes.