What is the difference between nuclease-free water and ultrapure water?

Nuclease-free water is free of both DNAse and RNAse, and involves treating with DEPC (diethylpyrocarbonate) and/or autoclaving to inactivate RNAse and DNAse.

Ultrapure water, on the other hand, is obtained by ultrafiltration to achieve the high level of purity. It also does not contain any nucleases.