Peroxidase is any of a group of enzymes which remove hydrogen peroxide (H2O2) from a cell by turning it into water and oxygen.
Hydrogen peroxide is a very harmful radical that, if left to its own molecular devices, will cause extensive damage to DNA. These enzymes are thus very important to all living things which use oxygen (aerobes) because the use of oxygen in respiration often results in the formation of hydrogen peroxide as a waste product. The formation of hydrogen peroxide is also a major reason why obligate anaerobes find oxygen to be a poison.
Here are a few peroxidases:
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.