What is the difference between the anti-bioterrorism organizations in the resident evil universe [BSA,D.S.O,TerraSave]?
Bioterrorism Security Assessment Alliance (BSAA) are a United Nations backed combat group who's main purpose is to prevent, and exterminate bio-terrorism.
Division of Security Operations (DSO) are similar to the BSAA but are under the direct jurisdiction of the US government.
TerraSave are primarily a human rights organization.
There's loads of info about each on their respective wiki entries, no point in copying and pasting it all here.