Small RNAs are noncoding RNA molecules between 20 and 200 nucleotide in length. The item "small RNA" is a rather arbitrary term, which is vaguely defined based on its length comparing with regular RNA such as messenger RNA (mRNA). Previously bacterial short regulatory RNAs have been referred to as small RNAs, but they are not related to eukaryotic small RNAs. 
Small RNAs include several different classes of noncoding RNAs, depending on their sizes and functions: snRNA, snoRNA, scRNA, piRNA, miRNA, YRNA, tsRNA, rsRNA, and siRNA. Their functions go from RNAi (specific for endogenously expressed miRNA and exogenously derived siRNA), RNA processing and modification, gene silencing (i.g. X chromosome inactivation by Xist RNA), epigenetics modifications, protein stability and transport.