H202(hydrogen peroxide) reacts with live tissue releasing nascent oxygen thereby allowing mechanical debridement of any necrotic tissues. its usually diluted with normal saline before using on tissues. can be used in areas where there is collection of pus or for removing any debris in areas of minor trauma due to its mechanical as well as its bactericidal action. but a major disadvantage is its easily decomposed by light.should be used under medical supervision.

saline on the other hand is milder. doesnt have very potent bactericidal effect yet very much used nowadays in the field of dentistry.can be used liberally.

hope this helps..if u have further qs do post it