BHO/BestOff is classified as a BHO, "browser helper object".
BHOs are Internet browser plug-ins that may perform malicious functions.
The Fortinet Anti-Spyware Research Team is currently in the process of creating a detailed description for this Spyware.
For additional information about BHOs, and how Fortinet classifies Spyware threats please see Fortinet's paper in PDF format:
Spyware Classification and Terms.