MicroRNAs are the most studied class of noncoding RNAs and range in size from 20-24 nt. miRNA gene are transcribed and form primary miRNA (pri-miRNA). Pri-miRNA are processed by Drosha and form precursor miRNA (pre-miRNA). A protein called exportin 5 exports pre-miRNA from the nucleus into the cytoplasm, where it is cleaved by an endonuclease called Dicer. After unwinding, single stranded miRNAs are formed and recruits the RNA-induced silencing complex (RISC) to a target mRNA for gene silencing.
See also this short video on Dicer and Drosha: http://www.scivee.tv/node/9346