Bacteriostatic Water is a sterile, nonpyrogenic preparation of water for injection but contains 0.9% of benzyl alcohol added as a bacteriostatic preservative to preserve the drug it is used with and also to prevent infections when needles are inserted multiple times like we see with HCG or Insulin.
So, bacteriostatic water is sterile water BUT contains a bacteriostatic preservative and more importantly an antimicrobial agent in it.
Sterile Water on the other hand is a sterile nonpyrogenic preparation of water for injection but contains no bacteriostat, antimicrobial agent or added buffer so it won't preserve the drug as long as Bacteriostatic water will or prevent infections from multiple insertions of needles.
Here's a good link:
http://www.rxlist.com/h2o-side-effects-drug-center.htm