A Ribonuclease (RNAse) is an enzyme that catalyzes the breakdown of RNA molecules into smaller components. RNAses are extremely common in the modern world, resulting in very short lifespans for any RNA that is not in a protected environment.
An RNAse that is commonly used in research is RNAse A. RNAse A (bovine pancreatic ribonuclease A: PDB 2AAS, EC 126.96.36.199) is one of the hardiest enzymes in common laboratory usage; one method of isolating it is to boil a crude cellular extract until all enzymes other than RNAse A are denatured.
RNase P, another type of RNase, is currently under much research and is unique in that it is a ribozyme, a ribonucleic acid that acts as a catalyst in the same way that a protein based enzyme would. Its function is to cleave off an extra, or precurser, sequence of RNA on tRNA molecules. RNase P has two components: an RNA chain, called M1 RNA, and a polypeptide chain, or protein, called C5 protein. In vivo, both components are necessary for the ribozyme to function properly, but in vitro, the M1 RNA can act alone as a catalyst.